U  C. 


EOCKS 

AND 

BOOK   MINEEALS 

A  MANUAL  OF  THE  ELEMENTS  OF  PETROLOGY 
WITHOUT  THE  USE  OF  THE  MICROSCOPE 

FOR  THE  GEOLOGIST,    ENGINEER,    MINER,   ARCHITECT, 

ETC.,   AND  FOR  INSTRUCTION  IN  COLLEGES 

AND    SCHOOLS 


BY 

LOUIS   V.  PIRSSON 

LATE    PROFESSOR   OF  PHYSICAL,  GEOLOGY   IN  THE   SHEFFIELD 
SCIENTIFIC    SCHOOL   OF  YALE  UNIVERSITY 


TOTAL  ISSUE  TWELVE  THOUSAND 


NEW  YORK 

JOHN  WILEY  &  SONS,  INC. 
LONDON:  CHAPMAN  &  HALL,  LIMITED 


OOPTBIQRT,   1906, 
BY 

LOUIS  V.  PIRSSON 
Entered  at  Stationers'  Ha 


Stanbopc  ipros 

H.G1LSON     COMPANY  1-25 

BOSTON.     U.S. A 


PREFACE. 


DURING  the  last  fifteen  years  it  has  been  one  of  the 
writer's  duties  to  teach  the  elements  of  Petrology  to 
students  in  various  branches  of  Engineering,  Mining, 
Chemistry,  Forestry,  etc.  The  amount  of  time  which 
these  students,  in  their  undergraduate  course,  can  devote 
to  the  subject  is  limited  and  precludes  any  attempt  to 
give  them  such  instruction  in  optical-microscopical 
methods  of  research  as  would  be  worth  while.  The 
subject  has  to  be  treated  from  the  purely  megascopic 
standpoint,  as  indeed  the  vast  majority  of  those  who 
have  to  deal  with  rocks  in  a  practical  or  technical  way 
are  also  obliged  to  consider  them. 

In  giving  this  instruction  the  author  has  long  felt  the 
need  of  a  small,  concise  and  practical  treatise  in  which 
the  rocks  and  rock-minerals  are  handled  entirely  from 
this  megascopic  standpoint.  In  such  works  as  exist 
either  the  subject  matter  has  not  been  brought  down  to 
date  to  express  our  present  knowledge  of  rocks,  or  it  is 
treated  largely  from  the  microscopical  or  chemical  stand- 
point, or  the  classifications  used  are  based  on  microscopical 
research  and  are  thus  not  available  for  ordinary  use,  or 
the  rocks  are  treated  incidentally  with  respect  to  some 
other  main  purpose  as  in  works  on  soils,  ore  deposits,  etc. 
The  present  work  is  an  attempt  to  fill  this  need  which  the 
writer  believes  is  also  felt  in  many  other  institutions  in 
which  similar  courses  in  Petrology  are  given.  In  addition 
to  this  purpose  its  scope  has  also  been  somewhat  enlarged 
to  meet  the  wants  of  many  who  have  to  consider  rocks 
from  the  scientific  or  practical  point  of  view  and  who  are 
not  in  a  position  to  use  the  microscopical  method.  It  is 
hoped  that  it  may  thus  prove  of  service  to  field  geologists, 

iii 


iv  PREFACE 

engineers,  chemists,  architects,  miners,  etc.,  as  a  handy 
work  of  reference.  Much  of  the  theoretical  side  of 
Petrology  which  has  been  developed  during  the  last  few 
years,  especially  in  the  line  of  petrogenesis,  does  not 
demand  a  knowledge  of  microscopical  petrography  for 
its  understanding,  and  the  endeavor  has  been  made  to 
present  the  elements  of  this  in  a  simple  manner.  Although 
the  author  has  incorporated  considerable  original  material 
it  goes  without  saying  that  a  work  of  this  character  must 
of  necessity  be  mainly  one  of  compilation.  It  would  be 
nearly  impossible  and  in  any  case  out  of  place  in  an 
elementary  treatise  to  give  by  reference  the  thousand 
and  one  sources  from  which  the  material  has  been  taken. 
It  should  be  mentioned,  however,  that  in  the  description 
of  the  minerals  the  writer  has  drawn  freely  upon  the 
mineralogies  of  Dana,  Iddings  and  Rosenbusch  and  the 
determinative  mineralogy  of  Brush-Penfield.  In  the  same 
way  in  the  treatment  of  the  rocks  the  petrographies 
of  Rosenbusch  and  Zirkel  and  the  geological  text-book 
of  Geikie  have  been  freely  used. 

Most  of  the  illustrations  have  been  prepared  for  this 
work,  but  the  wealth  of  material  in  the  published  reports, 
bulletins,  etc.,  of  the  United  States  Geological  Survey 
has  also  been  freely  used. 

SHEFFIELD  SCIENTIFIC  SCHOOL  OF  YALE  UNIVERSITY. 
New  Haven,  Conn.,  Jan.,  1908. 


TABLE  OF   CONTENTS. 


PART  I. 

Introductory  and  General  Considerations. 

Chapter  Page 

I.    SCOPE  OF  PETROLOGY;  HISTORICAL;  METHODS  OF  STUDY        1 
II.   CHEMICAL  CHARACTER  OF  THE  EARTH'S  CRUST  AND  ITS 

COMPONENT  MINERALS    .  .    .       14 


PART  II. 

Rock  Minerals. 

III.  IMPORTANT  PROPERTIES  OF  MINERALS 21 

IV.  DESCRIPTION  OF  THE  ROCK-MAKING  MINERALS     ...  33 
V.   DETERMINATION  OF  THE  ROCK-MAKING  MINERALS  .    ,  114 

PART   III. 
The  Rocks. 

VI.   GENERAL  PETROLOGY  OF  IGNEOUS  ROCKS 132 

VII.   DESCRIPTION  OF  IGNEOUS  ROCKS 205 

VIII.   ORIGIN  AND  CLASSIFICATION  OF  STRATIFIED  ROCKS    .  275 

IX.   DESCRIPTION  OF  STRATIFIED  ROCKS 293 

X.   ORIGIN,  GENERAL  CHARACTERS  AND  CLASSIFICATION  OP 

THE  METAMORPHIC  ROCKS 333 

XI.   DESCRIPTION  OF  METAMORPHIC  ROCKS 351 

XII.  THE  DETERMINATION  OF  ROCKS 398 

INDEX  .  409 


ROCKS  AND  ROCK  MINERALS 


PART  I. 

INTRODUCTORY  AND  GENERAL. 


CHAPTER  I. 

SCOPE  OF  PETROLOGY;  HISTORICAL;  METHODS 
OF  STUDY. 

EVERYWHERE  beneath  the  mantle  of  soil  and  vegetation 
that  covers  the  surface  of  the  land  lies  rock,  the  solid 
platform  upon  which  the  superficial  debris  of  earth  rests. 
Here  and  there  in  mountain  tops,  in  cliffs  and  ledges,  we 
see  this  underlying  rock  projecting  from  the  soil  and 
exposed:  we  know  that  it  must  underlie  the  sea  in  the 
same  way.  The  outer  shell  of  the  earth  then  is  made  of 
rock,  which  forms  the  foundation  upon  which  rest  all  the 
surface  things  with  which  we  are  acquainted.  How  thick 
this  zone  of  rock  is  we  do  not  know,  but  upon  it  we  live 
and  exert  our  activities;  into  it  we  penetrate  for  coal,  oil, 
gas,  metals  and  other  things  upon  which  the  material 
features  of  our  modern  civilization  depend.  It  is  there- 
fore of  the  highest  importance  to  us,  and  the  information 
which  we  have  acquired  concerning  it,  by  examination 
and  study,  forms  a  valuable  branch  of  human  knowledge. 

Petrology  —  the  Science  of  Rocks.  Our  knowledge  of 
all  the  various  things  which  together  make  up  that  part 
of  the  earth  which  it  is  permitted  us  to  examine  and  study 
and  which  has  been  comprehended  under  the  heading  of 
Geology  has  now  increased  to  such  a  degree  that  this 
science  has  split  up  into  a  number  of  well-defined,  sub- 
ordinate branches  or  geological  sciences.  Thus  Meteor- 
ology is  the  science  of  the  atmosphere,  the  summation  of 

1 


2  ROCKS  AND  ROCK  MINERALS 

our  knowledge  of  the  causes  and  movements  of  winds, 
storms,  rain,  the  distribution  of  heat  and  cold,  and  in 
general  the  study  of  the  various  factors  that  affect  the 
air  and  its  movements  and  of  the  laws  that  govern  them. 
Physiography  takes  account  of  the  surface  features  of 
the  earth,  of  the  distribution  of  land  and  water  and  of  the 
agencies  which  are  modifying  them,  the  effects  of  climates 
and  the  various  causes  which  together  produce  the  topo- 
graphy which  the  earth's  surface  now  exhibits.  Paleon- 
tology is  the  science  resulting  from  the  study  of  the  remains 
of  past  life  upon  the  earth,  as  shown  by  the  fossils  inclosed 
in  the  rocks,  and  teaches  not  only  the  different  forms 
which  have  existed  but  also  seeks  to  discover  the  trans- 
formation of  one  form  into  another  and  the  various  move- 
ments or  migrations  of  life  upon  the  earth  in  past  ages. 

Petrology,  in  the  same  way,  has  now  become  a  separate 
branch,  one  of  the  geologic  sciences.  It  comprises  our 
knowledge  of  the  rocks  forming  the  crust  of  the  earth,  the 
results  of  our  studies  of  the  various  component  materials 
which  form  them,  of  the  different  factors  and  the  laws 
governing  them  which  have  led  to  their  formation,  and 
of  their  behavior  under  the  action  of  the  agencies  to  which 
they  have  been  subjected,  and  endeavors  to  classify  the 
kinds  into  orderly  arrangement. 

The  terms  Petrology  and  Petrography  are  not  abso- 
lute synonyms  though  often  so  used  in  a  general  way. 
The  former  has  been  defined  above;  the  latter  more 
particularly  refers  to  the  description  of  rocks  and  especially 
with  respect  to  their  study  by  means  of  the  microscope 
as  explained  later  —  thus  microscopical  petrography. 
Petrology  is  used  for  the  science  in  its  broader  aspects  as 
well  and  covers  the  geological  and  chemical  relations  of 
rocks:  thus  strictly  defined  petrography  may  be  said  to 
be  a  branch  of  petrology.  The  synonym  Lithology  has 
become  nearly  obsolete.  Petrology  means  the  science  of 
rocks;  lithology,  the  science  of  stones,  and  the  word  stone 
is  now  used  in  a  popular  way  for  architectural  and  com- 


METHODS   OF  STUDY  3 

mercial  purposes  or  to  designate  any  loose  piece  of  rock 
of  unknown  origin. 

Definition  of  a  Rock.  By  the  term  "  rock,"  geologically 
speaking,  is  meant  the  material  composing  one  of  the 
individual  parts  of  the  earth's  solid  crust,  which,  if  not 
exposed,  everywhere  underlies  the  superficial  covering 
of  soil,  vegetation  or  water  which  lies  upon  it.  The 
popular  understanding  of  this  term,  that  it  denotes  a 
hard  or  firm  substance,  is  not,  geologically,  a  necessary 
one,  for  a  soft  bed  of  clay  or  of  volcanic  ash  is  as  truly  a 
rock  as  a  mass  of  the  hardest  granite.  Moreover  it 
implies  within  limits,  which  will  be  explained  elsewhere, 
a  certain  constancy  of  chemical  and  mineral  composition 
of  the  mass  recognized  as  forming  a  particular  kind  of 
rock.  Thus  the  chance  filling  of  a  mineral  vein  by  variable 
amounts  of  quartz,  calcite  and  ores  is  not  accepted  by 
petrographers  as  forming  a  definite  kind  of  rock.  The 
term  is  also  used  with  different  meanings;  it  may  be 
denotive  of  the  substance  forming  parts  of  the  earth's 
crust,  as  quartz  and  feldspar  arranged  in  a  particular 
manner  are  said  to  form  a  rock  —  granite  —  or  it  may 
refer  to  the  masses  themselves  and  thus  possess  a  larger, 
geological  significance.  In  a  general  way  the  former 
may  be  said  to  be  a  petrographic,  the  latter  a  geologic 
usage.  When  used  in  this  broader  geologic  sense  the 
mass  recognized  as  an  individual  kind  of  rock  must 
possess  definite  boundaries  and  show  by  its  relations  to 
other  rock  masses  that  it  owes  its  existence  to  a  definite 
geological  process.  The  absolute  size  of  the  mass  is  not 
involved  in  this,  for  a  seam  or  dike  of  granite  cutting  rocks 
of  other  kinds  may  be  as  thin  as  cardboard  or  a  mile  in 
thickness. 

Composition  of  Rocks.  Rocks  are  sometimes  defined 
as  aggregates  of  one  or  more  minerals,  but  this  is  not  a 
broad  enough  or  wholly  correct  definition.  Rocks  may 
be  composed  entirely  of  minerals  or  entirely  of  glass  or 
of  a  mixture  of  both.  Minerals  are  substances  having 


4  ROCKS  AND  ROCK  MINERALS 

definite  chemical  compositions  and  usually  of  crystalline 
structure;  glasses  are  molten  masses  chilled  and  solidified 
without  definite  composition  and  structure.  Rocks 
composed  wholly  of  minerals  may  be  simple  or  compound, 
that  is,  the  rock  may  be  formed  of  one  kind  of  mineral 
alone,  as  for  example,  some  of  the  purest  marbles  which 
consist  of  calcite  only  or  of  a  mixture  of  two  or  more  like 
ordinary  granite  which  is  made  of  grains  of  quartz,  feld- 
spar and  mica.  These  subjects  are  treated  more  fully  in 
later  chapters. 

History  of  Petrology.  The  science  of  geology  may  be 
said  to  have  commenced  when  rocks  as  objects  of  inves- 
tigation began  to  be  studied.  As  the  individual  minerals 
composing  rocks,  or  contained  in  their  cavities,  were 
investigated  by  chemical  means  and  by  the  goniometer, 
the  science  of  Mineralogy  and  its  related  subject,  Crys- 
tallography, began.  At  first  the  difference  between 
rocks  and  minerals  was  not  very  clearly  perceived;  very 
dense  rocks  composed  of  mineral  grains  so  fine  that  they 
could  not  be  distinguished  by  the  eye  or  magnifying  lens 
were  thought  to  be  homogeneous  substances,  and  similar 
in  their  nature  to  minerals.  This  continued  in  many 
cases  even  down  to  the  middle  of  the  last  century. 

As  the  knowledge  of  the  composition  and  properties  of 
minerals  grew  it  was  seen  in  the  case  of  the  coarser 
grained  rocks  that  they  were  composed  of  aggregates  of 
these  mineral  grains,  and  according  to  the  kinds  of  the 
component  minerals  many  common  rocks  had  already 
received  names  early  in  the  last  century.  As  investiga- 
tion proceeded  and  geological  science  grew  many  new 
combinations  were  discovered  and  the  list  of  named 
rocks  increased,  and  it  may  be  remarked  here  that  these 
early  geologists,  armed  only  with  a  simple  lens,  were 
exceedingly  keen  observers  and  made  many  surprisingly 
correct  observations  on  the  mineral  composition  of  quite 
fine  grained  rocks.  Various  schemes  of  classification  were 
proposed,  some  of  them  containing  admirable  features, 


METHODS  OF  STUDY  5 

but  the  dense  varieties  defied  the  means  of  investigation 
then  at  command,  and  in  great  part  their  composition, 
properties  and  relations  to  other  rocks  remained  unknown. 
About  the  middle  of  the  last  century  Sorby,  an  English 
geologist,  showed  that,  by  a  suitable  method  of  operation, 
very  thin  slices  of  rocks  could  be  prepared,  and  by 
the  study  of  such  thin  sections  under  the  microscope  the 
kinds  of  component  mineral  grains  could  be  made  out, 
their  properties  and  relations  to  one  another,  the  order  in 
which  they  had  been  formed,  the  processes  to  which  they 
had  been  subjected,  and  many  other  interesting  and 
important  features  discovered,  and  that  it  was  possible 
to  do  this  even  in  the  case  of  the  densest  and  most  com- 
pact rocks.  This  method  of  investigation  was  imme- 
diately taken  up,  especially  in  Germany  by  Zirkel  and 
others,  and  with  its  advent  a  new  era  in  the  study  of  rocks 
and  the  science  of  Petrography  may  be  said  to  have 
begun.  A  flood  of  knowledge  regarding  rocks  and 
especially  of  the  minerals  composing  them  began  to  rise 
and  has  kept  on  increasing  to  the  present  day.  The 
study  of  the  properties  of  transparent  minerals  under  the 
action  of  polarized  light  received  a  great  impulse,  and 
the  facts  discovered  have  in  turn  been  of  immeasurable 
service  in  the  investigation  of  rocks  by  this  method. 
Thus  at  first  attention  was  directed  chiefly  to  the  mineral- 
ogical  side  of  petrology;  the  kinds  of  minerals  of  which 
rocks  are  composed  and  their  properties  were  considered 
of  first  importance,  and  this  is  reflected  in  the  schemes  of 
classification  devised  at  this  period.  Later,  the  chemical 
composition  of  rocks,  both  in  mass  and  as  shown  in  their 
component  minerals,  their  origin  and  the  relations  of  the 
different  varieties  to  each  other  began  to  attract  more 
attention  and  have  been  regarded  as  of  increasing  impor- 
tance down  to  the  present  day.  This  increasing  impor- 
tance of  these  aspects  of  the  subject  is  also  seen  in  the 
weight  placed  upon  them  in  the  more  recent  schemes  of 
classification  proposed  for  the  igneous  rocks,  those  formed 


6  ROCKS  AND  ROCK  MINERALS 

by  the  solidification  of  the  molten  masses  coming  from 
the  earth's  interior. 

Classification  of  Rocks.  According  to  their  mode  of 
origin  and  the  position  of  the  masses  with  respect  to  the 
earth's  crust  and  to  each  other,  rocks  naturally  divide 
themselves  into  three  main  groups,  divisions  which  are 
recognized  by  practically  all  geologists.  These  are  the 
igneous  rocks  made  by  the  solidification  of  molten  material; 
the  sedimentary  or  bedded  rocks  formed  by  the  precipita- 
tion of  sediments  in  water,  to  which  may  be  added  the 
small  group  of  seolian  or  wind  formed  deposits,  and  the 
metamorphic  rocks,  those  produced  by  the  secondary 
action  of  certain  processes  upon  either  igneous  or  sedi- 
mentary ones  by  which  their  original  characters  are 
wholly  or  partly  obscured  and  replaced  by  new  ones  and 
which  are  therefore  most  conveniently  considered  in  a 
separate  group.  This  grouping  will  be  used  in  this  work, 
and  each  group  with  its  further  subdivisions,  their  char- 
acters, relations,  etc.,  will  be  treated  by  itself. 

Summarizing  then  what  has  just  been  stated,  we  have: 

I.     Igneous  Rocks,  solidified  molten  masses. 
II.     Sedimentary  Rocks,  precipitated  sediments. 
III.     Metamorphic  Rocks,  secondary  —  formed  from 
I  and  II. 

Field  and  Petrographic  Classifications.  The  sedimentary 
rocks  are  classified  in  two  ways,  —  in  one  they  are  sub- 
divided according  to  the  kinds  and  fineness  of  the  mineral 
particles  which  compose  them,  in  the  other  according  to 
the  geological  age,  as  shown  by  their  position  and  fossils, 
in  which  the  sediments  were  laid  down.  The  first  is  the 
petrographic,  the  second  the  geological,  or  more  strictly 
the  historical  classification,  and  in  this  work  these  rocks 
are  treated  only  according  to  the  former  method.  In 
classifying  them  they  have,  so  far,  been  simply  divided 
according  to  the  properties  mentioned  above,  and  as  they 
have  not  yet  been  the  subject  of  the  detailed  research 


METHODS   OF   STUDY  7 

which  their  importance  demands,  the  simple  classifica- 
tion adopted  by  geologists  in  the  field  has  been  followed 
by  the  petrographers. 

With  respect  to  the  igneous  rocks  and  to  a  lesser  degree 
the  metamorphic  ones  the  case  is  different.  The  use  of 
the  microscope  in  the  study  of  thin  sections  has  shown 
that  rocks  which  may  appear  absolutely  identical,  either 
in  the  field  or  as  one  simply  compares  hand-specimens, 
may  be  composed  of  entirely  different  minerals,  or  their 
chemical  analysis  may  prove  them  to  be  fundamentally 
different  in  chemical  composition.  They  may  thus  be 
quite  different  rocks  deserving  separate  names  and  places 
in  any  classification  in  which  all  of  the  essential  char- 
acters of  rocks  are  considered,  and  yet  outwardly  to  the 
eye  there  may  be  no  hint  of  this.  There  have  arisen  two 
useful  terms,  megascopic  (from  the  Greek  ju,eyas  —  great) 
and  microscopic,  the  first  descriptive  of  those  characters 
of  rocks  which  can  be  perceived  by  the  eye  alone  or  aided 
by  a  simple  pocket-lens,  and  the  second  referring  to  those 
which  require  the  use  of  the  microscope  on  thin  sections. 
It  is  obvious  that  a  classification  which  is  based  upon 
microscopic  characters  as  much  as  upon  megascopic  ones 
cannot  be  used  in  determining  rocks  in  the  field.  It  may 
be  more  correct  and  scientific,  but  in  the  nature  of  things 
it  cannot  be  of  general  application  and  use.  This  subject 
will  be  treated  of  more  fully  in  the  section  devoted  to  the 
igneous  rocks,  and  it  is  sufficient  to  say  here  that  the 
object  of  this  work  is  to  supply  a  field  classification 
based  upon  the  megascopic  characters  of  rocks  to  be 
determined  by  the  eye  or  pocket-lens,  aided  by  a  few 
simple  means  for  the  determination  of  minerals.  In 
addition  many  important  facts  regarding  rocks  and 
especially  igneous  ones  have  been  discovered  in  these 
later  years  which  are  not  dependent  upon  their  classifica- 
tion or  microscopic  study,  and  it  is  intended  to  give  some 
account  of  these  in  a  simple  general  way. 

Microscopical  Petrography.     Although  this   volume  is 


8 


ROCKS  AND  ROCK  MINERALS 


not  based  upon  the  microscopical  method  of  research  it 
will  be  of  interest  to  indicate  briefly  how  this  is  conducted 
and  the  sort  of  results  obtained  by  it.  To  prepare  the 
thin  sections  or  slices  of  rock  for  study,  a  chip  of  the 
material  as  thin  as  can  be  obtained  is  taken.  It  should 
be  for  ordinary  purposes  about  an  inch  in  diameter  and 
of  firm  unaltered  material.  It  is  first  ground  flat  on  a 
metal  plate  with  coarse  emery  powder  and  water  and  then 
very  smooth  on  another  plate  with  very  fine  powder.  It 
is  then  cemented  by  the  aid  of  heat  to  a  piece  of  glass 
with  Canada  balsam  and  the  other  side  ground  down  with 
the  coarse  emery  until  it  is  as  thin  as  cardboard,  or  as 
far  as  it  is  possible  to  carry  the  operation  safely  with  the 
coarse  powder.  It  is  then  in  a  similar  way  ground  down 
with  the  finer  powder  and  finally  finished  on  a  glass  plate 
with  the  finest  flour  of  emery  until,  in  the  case  of  dark, 
dense  rocks,  it  becomes  so  thin  that  ordinary  print  may 
be  read  through  it.  It  is  then  transferred,  after  melting 
the  cementing  balsam,  to  a  microscopic  object  glass  slide, 
enveloped  in  balsam,  a  thin  cover  glass  placed  over  it,  and 
it  is  then  ready  for  use.  The  professionals  who  make  a 
business  of  preparing  such  sections  save  much  time  in  the 
coarser  work  by  the  use  of  sawing  disks  with  diamond 
dust  embedded  in  them  or  by  using  car- 
borundum powder  on  disks  or  endless 
revolving  wires.  They  become  very  expert 
in  the  final  grinding  and  prepare  sections 
whose  thickness  is  quite  uniformly  about 
WITTJ  °f  an  inch,  experience  having  shown 
that  this  is  best  for  general  work.  The 
appearance  of  one  of  these  finished  sections 
is  shown  in  Figure  1.  By  this  process  the 
minute  mineral  grains  composing  the 
densest  and  blackest  of  basaltic  lavas 
become  transparent  and  may  be  deter- 
mined under  the  microscope. 
The  microscope  used  in  petrographic  work  differs  from 


Fig.  i.  Thin  rock 
section 


PLATE    1. 


METHODS  OF  STUDY  9 

that  ordinarily  employed  in  being  furnished  with  a 
variety  of  apparatus  arranged  for  studying  the  mineral 
sections  in  polarized  light.  Underneath  the  table  or 
stage  which  carries  the  section  is  a  nicol  prism  of  calcite 
which  polarizes  the  light  coming  upward  from  the  reflect- 
ing mirror  below  before  it  passes  through  the  section, 
that  is,  the  vibrations  of  the  ether  which  produce  the 
phenomenon  of  light  instead  of  occurring  in  all  directions, 
as  in  common  light,  are  reduced  to  a  definite  direction  in 
one  plane. 

Another  nicol  prism  called  the  analyzer  is  fitted  in  the 
tube  above  the  object  lens  so  that  the  effects  produced  by 
the  mineral  particles  on  the  polarized  light,  as  it  passes 
through  them,  can  be  tested  and  studied.  The  nicols 
can  be  also  removed  so  that  the  effects  of  ordinary  light  can 
be  seen.  Other  arrangements  are  provided  for  strongly 
converging  the  light  as  it  passes  through  the  minerals 
and  for  testing  the  results  produced  in  a  variety  of  ways. 

Subjected  to  such  processes  the  transparent  minerals 
of  the  rocks  exhibit  a  great  variety  of  phenomena  by 
means  of  which  the  different  species  may  be  definitely 
determined.  Crystals  or  fragments  of  crystals  of  an 
almost  incredible  degree  of  minuteness  may  be  studied 
with  high  powers,  their  properties  examined  and  the  par- 
ticular variety  of  mineral  to  which  they  belong  made  out. 
In  order  to  use  this  means  of  studying  rocks  a  good 
knowledge  of  general  Mineralogy,  of  Crystallography,  of 
Optics,  and  in  particular  of  the  optical  properties  of  the 
rock-making  minerals  is  essential.  Owing  to  the  cost  of 
the  apparatus,  the  technical  knowledge  required  in  its 
use  and  the  difficulty  of  making  thin  sections,  it  is  obvious 
that  this  method  of  studying  rocks  can  never  become  a 
popular  or  general  one,  but  many  of  the  results  which 
have  been  attained  by  it  are  easily  understandable  and 
may  be  mentioned. 

Results  of  Microscopic  Research.  By  the  method 
described  above  the  kind  of  mineral  or  mineral  grains 


10  ROCKS  AND   ROCK    MINERALS 

making  up  the  most  compact  and  dense  rocks  may  be 
determined;  whether  the  rock  is  of  sedimentary  or  igneous 
origin  can  be  told  and,  if  the  latter,  the  general  order  in 
which  the  mineral  varieties  have  crystallized  from  the 
molten  fluid.  It  can  be  seen  whether  the  crystal  particles 
are  clear  and  homogeneous  or  if  they  contain  inclusions 
of  various  kinds,  facts  which  often  throw  light  on  their 
origin  and  history;  whether  they  are  fresh  and  unchanged 
or  have  been  decayed  by  the  action  of  the  elements  and 
altered  wholly  or  partly  into  other  substances;  whether 
they  have  been  subjected  to  the  enormous  pressures  of 
mountain  building  in  the  crust  and  are  strained  and 
crushed  or  not.  It  is  possible  to  tell  at  once  if  a  rock 
contains  more  or  less  glass  associated  with  the  mineral 
grains,  and  if  it  does,  to  thus  learn  with  certainty  its 
igneous  origin  and  the  fact  that  in  all  probability  it  is  a 
surface  lava,  —  glass,  in  the  nature  of  things,  being  almost 
entirely  confined  to  such  rocks. 

Furthermore,  if  the  grains  are  not  too  microscopically 
fine  it  may  be  possible,  not  only  to  determine  the  kinds  of 
minerals  they  are,  but  to  measure  their  areas  or  diameters 
in  a  given  section,  ascertain  from  this  the  relative  pro- 
portions of  the  different  kinds  of  grains  present,  and  then, 
when  the  chemical  composition  of  the  component  minerals 
is  known,  compute  the  chemical  composition  of  the  rock 
mass  as  a  whole,  a  factor,  especially  in  the  case  of  igneous 
rocks,  often  of  great  importance  in  scientific  classification 
and  in  other  ways. 

These  are  some  of  the  more  important  features  of 
rocks  which  may  be  discovered  by  their  microscopic 
study,  and  they  are  sufficient  to  illustrate  the  value  of  the 
method  in  aiding  geological  research.* 

*  The  following  works  in  which  rock-making  minerals  and  rocks 
are  treated  and  classified  according  to  the  results  of  microscopical 
research  in  a  more  or  less  extensive  and  detailed  way  may  be  men- 
tioned: Rock  Minerals,  by  J.  P.  Iddings  (Wiley  and  Sons,  New  York). 
Quantitative  Classification  of  Igneous  Rocks,  by  Messrs.Cross,Iddings, 


METHODS  OF  STUDY  11 

Megascopic  Study  of  Rocks.  Although  the  microscope 
is  necessary  for  the  complete  investigation  of  rocks  many 
of  their  important  features  may  be  observed  without  its 
use.  In  the  case  of  the  coarser  grained  ones,  those 
where  the  size  of  the  grains  is  one-sixteenth  of  an  inch  in 
diameter  or  more,  the  component  minerals  can  usually 
be  identified  by  the  aid  of  the  lens  or  by  simple  means. 
Even  when  much  finer  grained  than  this,  some  minerals 
may  be  distinguished  by  certain  characters  they  possess 
as  explained  in  the  chapter  on  the  rock-making  minerals. 
Even  when  they  are  so  dense  that  the  component  grains 
can  no  longer  be  discriminated  from  each  other,  the  color, 
the  hardness,  the  style  of  fracture  under  the  hammer,  the 
specific  gravity  and  the  behavior  of  fragments  or  of  the 
powdered  rock  under  the  action  of  acids,  are  all  impor- 
tant characters  which  serve  to  distinguish  different  kinds 
of  rocks. 

Implements  and  Apparatus.  The  first  requisite  is  a 
suitable  hammer  for  obtaining  material.  It  should  be  a 
square-faced  geological  hammer  of  the 
type  shown  in  the  adjoining  figure.  It  is 
convenient  to  have  one  end  wedge  shaped. 
The  steel  should  be  tempered  as  hard  as 
possible  without  making  it  too  brittle, 
otherwise  the  edges  wear  off  very  rapidly. 
If  made  to  order  it  is  a  great  convenience 
to  have  the  hole  as  large  as  possible, 
consistent  with  strength,  and  tapered  some- 
what; the  handle  may  then  be  somewhat 
larger  at  the  hammer  end  and  thrust  Fig.  a.  Geological 
through  the  hole  until  brought  up  in  the 
taper  by  the  enlarged  end.  This  device,  which  is  the 
familiar  one  used  in  securing  the  handles  of  picks,  is  a  great 
convenience  as  it  prevents  the  head  coming  off  when  the 

Pirsson  and  Washington  (Chicago  University  Press).  Elemente  der 
Gesteinslehre,  by  H.  Rosenbusch  (Stuttgart).  Petrology  for  Students, 
by  A.  Harker  (Cambridge  University  Press).  Igneous  Rocks,  by 
J.  P.  Iddings  (Wiley  &  Sons,  New  York). 


12  ROCKS  AND  ROCK  MINERALS 

handle  shrinks.  The  hammer  should  be  of  good  weight, 
about  two  and  one-half  pounds  for  the  head,  to  enable 
good-sized  pieces  of  rock  to  be  readily  broken  up  and 
fresh  material  within  to  be  secured.  Of  course  anything 
in  the  way  of  a  hammer  or  sledge  may  be  used  on  occa- 
sion, but  this  implement  will  give  the  best  service  for 
general  use. 

If,  in  addition  to  procuring  material  for  examination, 
it  is  desired  to  trim  and  shape  it  into  specimens  for  the 
collection  a  small  trimming  hammer  will  be  found 
convenient.  It  should  be  double-headed, 
square-faced,  and  of  very  hard  steel,  and 
the  head  may  weigh  about  six  ounces.  Hand 
specimens  for  collections  are  usually  about 
4X3X1  inches  in  size  and  are  made  by 
selecting  a  suitable  large  flake  or  spall  obtain- 
ed by  the  large  hammer  and  knocking  small 
chips  from  it  along  the  edges  first  on  one  side 
.a.  Trimming  and  then  on  the  other  until  trimmed  to  the 
required  shape  and  size.  A  well-made 
specimen  should  show  hammer  marks  only  on  the  edges 
and  never  on  the  faces. 

A  pocket-lens  is  also  essential;  one  of  the  apochromatic 
triplets  now  made  by  several  makers  of  optical  instru- 
ments is  best,  but  much  cheaper  ones  will  serve  the 
purpose.  One  with  a  focal  distance  of  one  inch  is  most 
convenient  for  general  use. 

In  addition  to  the  above,  which  are  for  use  in  the  field, 
a  small  amount  of  the  apparatus  used  in  the  laboratory 
for  the  determination  of  minerals  will  often  prove  of 
great  service.  This  would  include  a  blowpipe  and  plati- 
num tipped  forceps  for  testing  fusibility,  pieces  of  quartz, 
calcite  and  ordinary  window  glass  for  testing  hardness,  a 
simple  apparatus  for  determining  specific  gravity,  a 
magnet,  a  few  test  tubes,  dilute  acids  and  a  Bunsen  gas 
burner  or  alcohol  lamp  for  testing  solubility,  and  a  glass 
funnel,  filter  paper  and  a  few  reagents,  such  as  solutions 


METHODS  OF  STUDY  13 

of  ammonia,  silver  nitrate  and  barium  chloride,  for 
making  tests  by  chemical  reactions.  A  small  agate  or 
steel  mortar  is  needed  for  grinding  a  fragment  of  the  rock 
or  mineral  to  powder  for  making  chemical  tests.  This 
list  might  be  increased  almost  indefinitely  into  the  full 
equipment  of  a  mineralogical  laboratory,  but  most 
chemical  laboratories  contain  all  the  apparatus  and 
reagents  necessary  for  the  determination  of  minerals  and 
rocks  mentioned  in  this  book  and,  where  such  a  laboratory 
is  not  available,  the  material  which  has  been  named  above 
will  cover  nearly  all  necessary  demands  and  may  be  used 
almost  anywhere. 


CHAPTER  II. 

CHEMICAL  CHARACTER  OF   THE  EARTH'S   CRUST  AND 
ITS  COMPONENT  MINERALS. 

Composition  of  the  Earth's  Interior.  The  origin  and 
history  of  the  rocks  composing  the  solid  crust  of  the  earth 
are  of  necessity  bound  up  with  the  history  and  origin  of 
the  globe  itself.  Beyond  that  history,  however,  which  is 
revealed  in  the  sedimentary  rocks,  our  ideas  on  this  sub- 
ject, as  regards  the  earth,  must,  with  our  present  knowl- 
edge, be  largely  of  the  nature  of  pure  speculation.  Below 
a  relatively  very  shallow  depth  the  same  is  true  with 
respect  to  the  character  and  condition  of  its  interior.  We 
do  not  know  what  it  is  like,  and  it  is  of  course  possible  that 
we  never  shall.  The  view  which  is  most  generally  held 
is  that  the  earth  was  once  a  molten  mass,  the  outer  shell 
of  which  solidified  through  cooling  to  a  solid  crust,  while 
the  interior,  though  excessively  hot,  also  solidified  through 
the  enormous  pressures  exerted  upon  it  by  the  overlying 
portions;  between  the  two  is  either  a  zone  of  liquid  because 
the  pressure  is  not  there  sufficient  to  solidify  it,  or  of 
heated  material  which  will  become  liquid  if  for  any  cause 
the  pressure  in  the  crust  above  is  diminished;  this  zone  is 
regarded  as  the  seat  of  volcanic  and  other  important 
geological  activities. 

While  this  view  has  been  long  and  is  still  widely  held 
and  has  been  of  great  service  in  explaining  many  geological 
phenomena,  certain  objections  to  it  have  been  advanced, 
and  recently  Chamberlain  has  propounded  another. 

According  to  this  the  earth  is  regarded  as  never  having 
been  liquid  but  always  a  solid  which  has  been  gradually 
built  up  by  the  infall  and  accretion  of  relatively  small 
solid  bodies  termed  planetesimals.  Through  the  enor- 

14 


CHARACTER  OF  THE  EARTH'S  CRUST  15 

mous  pressures  exerted  under  the  influence  of  gravity, 
contraction  has  ensued  and  gaseous  matters  have  been 
expelled,  giving  rise  to  the  atmosphere  and  water  on  the 
surface.  This  contraction  is  held  to  be  the  source  of  the 
interior  heat,  and  to  the  issuance  of  the  heated  gases  is 
attributed  the  origin  of  volcanic  activity. 

Still  another  view  has  been  advanced  in  recent  years  by 
Arrhenius  according  to  which  the  interior  is  neither  in  a 
solid  or  liquid  but  in  a  gaseous  condition.  It  is  assumed 
that  all  substances  if  sufficiently  heated  must  be  in  the 
state  of  a  gas;  experiment  teaches  us  that  if  any  gas  is 
heated  to  or  above  a  certain  degree  called  its  "  critical 
temperature  "  it  cannot  be  reduced  to  a  liquid  or  solid  by 
pressure  alone,  and  it  is  held  that  this  will  be  true  even 
though  the  pressure  be  enormous  enough  to  contract  the 
gases  to  a  density  far  beyond  that  which  the  substances 
would  have  if  in  the  solid  condition.  It  is  assumed  that 
the  temperatures  reigning  in  the  earth's  interior  are  so 
great  that  all  substances  must  be  in  a  gaseous  condition 
and  above  their  critical  temperatures,  but  that  the 
pressures  are  also  so  enormous  that  they  are  reduced  to  a 
state  of  density  far  greater  than  that  of  solids  at  the 
surface,  and  that  on  account  of  this  condensation  their 
internal  viscosity  or  resistance  to  flowage  is  so  great  that 
they  possess  also  a  greater  rigidity,  one  sufficient  to  meet 
the  demand  which  astronomical  investigations  have  shown 
that  the  earth  as  a  whole  must  possess. 

Following  this  view  then  there  is,  first,  a  solid  outer 
crust,  then  a  zone  of  molten  liquid  or  of  solid  material  so 
greatly  heated  that  it  is  capable  of  becoming  liquid  if  the 
pressure  above  is  in  any  way  lightened,  and  then  finally 
the  great  interior  mass  consisting  of  heated  gases  in  a 
condition  of  enormous  condensation. 

The  three  hypotheses  presented  above  will  serve  to 
show  how  widely  divergent  are  the  views  in  regard  to  this 
subject  among  scientific  men  at  the  present  time  and  how 
purely  speculative  our  ideas  must  be. 


16  ROCKS  AND  ROCK  MINERALS 

Facts  which  are  known.  On  the  other  hand  it  must 
not  be  assumed  that  nothing  is  known  of  the  earth  beyond 
that  which  we  can  see  at  the  surface.  We  know,  for 
instance,  that  there  is  a  considerable  increase  in  heat  as 
we  go  downward  in  the  crust.  We  know  also  that  there 
are  bodies  of  molten  material,  which,  though  they  may 
be  relatively  small  as  compared  with  the  size  of  the  earth, 
are  yet  absolutely  large,  and  we  see  the  upward  prolonga- 
tion to  the  surface  of  these  masses  in  active  volcanoes. 
We  know  that  such  bodies  not  only  exist  in  the  earth's 
interior  now  but  have  also  in  past  geological  ages,  as 
shown  by  the  way  in  which  they  have  been  forced  upward 
into  its  crust  or  poured  out  upon  its  surface.  We  know 
that  upon  the  land  surfaces  wherever  the  deepest  seated 
rocks,  which  underlie  all  the  stratified  and  metamorphic 
ones  which  have  accumulated  upon  them,  are  exposed  by 
erosion  they  present  the  general  characters  of  igneous 
rocks,  and  thus  lead  us  to  infer  that  they  were  at  one  time 
in  a  state  of  fusion.  As  the  sedimentary  and  metamorphic 
rocks  are  secondary  or  derived  from  previously  existent 
ones,  this  leads  to  the  natural  assumption  that  they  came 
from  material  originally  similar  to  these  deep-seated  ones 
and  that  their  substance  had  at  some  previous  stage 
passed  through  a  state  of  fusion. 

Rock  material  then  having  been  wholly  or  at  least  very 
largely  in  a  molten  condition,  it  is  evidently  a  matter  of 
importance  that  we  should  know  something  of  the  nature 
and  properties  of  the  molten  fluids  which  have  formed 
it.  We  can  do  this  to  some  extent  in  active  volcanoes 
where  we  see  some  of  the  properties  of  these  fluids  exhib- 
ited, but  those  which  are  most  important  in  rock  forma- 
tion we  can  best  learn  by  study  of  the  igneous  rocks  which 
are  the  result  of  the  direct  solidification  of  these  molten 
masses,  and  this  subject  is,  therefore,  treated  in  the 
chapter  upon  them.  There  are,  however,  certain  aspects 
of  it  which  can  well  be  considered  here,  and  one  of  these 
is  the  general  chemical  composition  of  the  earth's  crust. 


CHARACTER  OF  THE  EARTH'S  CRUST  17 

Chemical  Composition  of  the  Earth's  Crust.  During 
recent  years  several  thousand  chemical  analyses  have 
been  made  of  rock  specimens  from  visible  parts  of  the 
earth's  crust.  The  great  majority  of  them  are  from 
Europe  and  the  United  States,  but  enough  have  been 
made  from  other  parts  of  the  world  to  show,  in  con- 
junction with  the  microscopical  studies  of  other  specimens, 
that  the  essential  facts  which  these  analyses  teach  are 
almost  beyond  question  of  general  application.  One  of  the 
most  important  general  truths  learned  by  these  investiga- 
tions may  be  thus  broadly  stated  —  the  general  chemical 
composition  of  the  earth's  crust  is  everywhere  similar. 

The  statement  thus  broadly  made  demands  explanation. 
It  does  not  mean  that  one  portion  of  the  rock  crust  is 
composed  of  exactly  the  same  chemical  elements  in 
exactly  the  same  proportions  as  any  other  portion.  It 
means  that  it  is  composed  of  the  same  elements  and  that, 
although  these  may  vary  greatly  in  proportions  from 
place  to  place  or  from  one  kind  of  rock  mass  to  another, 
if  we  take  large  areas  involving  many  kinds  of  rocks  the 
average  of  such  areas  will  be  very  nearly  alike.  Thus 
the  composition  of  the  average  rock  computed  from  all 
the  analyses  made  of  specimens  from  the  United  States 
is  essentially  the  same  as  the  average  computed  from  the 
analyses  of  the  rocks  of  Europe.  The  average  rock  of 
New  England  is  essentially  that  of  the  Rocky  Mountains 
region.  On  the  other  hand  a  large  part  of  Quebec  Province 
is  composed  of  one  kind  of  rock  which  extends  with 
monotonous  sameness  over  a  vast  area;  the  composition 
of  this  has  not  the  same  proportions  as  the  average  rock, 
and  if  we  were  considering  this  particular  part  of  the 
continent  we  should  have  to  increase  greatly  our  area  to 
obtain  an  average.  Some  parts  of  the  continental  areas 
are  covered  with  limestone  which  is  essentially  carbonate 
of  lime  alone,  but  is  a  relatively  thin,  concentrated  coating 
of  a  special  substance  —  we  should  have  to  balance  it  with 
large  masses  of  other  rocks. 


18 


ROCKS  AND  ROCK  MINERALS 


The  average  rock  has  been  computed  from  the  analyses 
by  Clarke  and  by  Washington  and  the  results  are  shown 
in  the  table  below  in  Column  A. 


A 

93  Per  Cent 
Lithosphere. 

B 

7  Per  Cent 
The  Ocean. 

C 

Average  with 
Atmosphere. 

Oxveen 

47  07 

85.79 

49  77 

Silicon      

28.06 

26  08 

Aluminum  

7.90 

7  34 

Iron  

4.43 

4  11 

3.44 

0.05 

3.19 

Magnesium     

2.40 

0.14 

2  24 

Sodium    

2.43 

1.14 

2  33 

Potassium  

2.45 

0.04 

2  28 

Hydrogen    

0.22 

10.67 

0.95 

Titanium     

0.40 

0.39 

Carbon     

0.20 

0.002 

0  18 

Chlorine  

0.07 

2.07 

0  21 

Phosphorus    

0.11 

0.10 

Sulphur   .....       .   . 

0  11 

0  09 

0  10 

All  others    

0  71 

0  008 

0  73 

100.00 

100.000 

100.00 

Clarke  has  also  calculated  that  if  we  assume  that  the 
crust  has  this  composition  to  a  depth  of  ten  miles  and 
add  in  the  water  of  the  oceans,  the  atmosphere  and  an 
amount  of  sedimentary  rocks  in  proper  proportions,  the 
general  average  of  the  whole  will  be  that  shown  in  Column 
C,  of  the  above  table.  These  assumptions  are  reasonable, 
and  correspond  with  the  facts  so  far  as  known.  Even  if 
these  results  are  not  very  accurate  they  must  be  approxi- 
mately so  and  they  are  of  value  in  showing  the  relative 
proportions  of  the  elements  in  the  outer  part  of  the  earth. 
From  them  important  deductions  can  be  drawn. 

The  Elements  of  Geological  Importance.  From  the 
table  just  given  we  see  that  the  first  eight  elements  are 
present  in  quantity,  and  are,  therefore,  of  geologic  impor- 
tance. Oxygen  forms  about  one  half  of  the  outer  part  of 


CHARACTER  OF  THE  EARTH'S  CRUST  19 

the  earth,  and  the  quantity  in  the  atmosphere  and  in  the 
ocean  is  small,  compared  with  that  locked  up  in  the  under- 
lying rock.  Silicon  comes  next  and  forms  about  one 
fourth  and  after  it  are  aluminium  and  five  other  metals,  of 
which  iron  is  the  most  important,  the  others  being  calcium, 
magnesium,  sodium  and  potassium,  in  the  order  of  their 
importance.  After  these  comes  a  small  group  of  four 
elements,  which,  although  of  secondary  rank  in  quantity, 
demand  mention:  they  are,  hydrogen,  titanium,  carbon 
and  chlorine.  Of  these,  titanium  is  a  rather  inert  element 
from  the  geological  standpoint,  plays  no  important  part 
in  geological  processes  or  results,  and  may,  therefore,  be 
dismissed.  The  hydrogen  and  carbon,  on  the  contrary, 
are  of  great  importance,  they  are  of  great  activity  in 
geological  processes,  produce  results  of  petrologic  interest, 
and  must  therefore  be  considered  with  the  primary  group 
first  mentioned.  All  the  other  elements,  however  impor- 
tant in  special  cases,  or  for  organic  life  or  civilized  activities, 
are  from  the  standpoint  of  general  geology  and  petrology 
of  relatively  little  interest. 

Combinations  of  Chemical  Elements.  Except  oxygen, 
carbon,  and  possibly  to  an  unimportant  extent  iron,  the 
elements  mentioned  above  do  not  occur  alone,  or  native; 
they  are  always  combined  in  some  form  producing  com- 
pounds known  as  minerals.  We  may  state  this  chemically 
by  saying  that  they  are  either  in  combination  with  oxygen 
as  oxides,  or  these  oxides  are  in  combination  as  salts. 
Two,  carbon  and  silicon,  are  negative  elements  —  their 
oxides  C02  and  Si02  are  anhydrides  of  acids;  the  others, 
leaving  hydrogen  aside,  are  metals,  or  positive  elements 
whose  oxides  act  as  bases.  The  oxide  of  aluminium, 
A1203,  acts  sometimes  as  a  base  and  sometimes  as  a  weak 
acid,  especially  in  combination  with  strong  positive  bases, 
such  as  soda,  Na2O,  and  potassa,  K2O,  and  in  combination 
with  silica,  SiO2.  Fe2O3  acts  as  an  acid  in  spinels. 

From  what  has  been  said  we  have  to  deal  with  these 
oxides  which  are  of  chief  petrologic  importance. 


20         ROCKS  AND  ROCK  MINERALS 


ACIDIC   OXIDES. 

SiO2,  silica,  in  combination  and  free  as  a  solid. 
CO2,  carbon  dioxide,  in  combination  and  free  as  gas. 

BASIC  OXIDES. 

AljOa,  alumina,  in  combination  and  free  solid,  sometimes  acidic. 

Fe2O3,  ferric  oxide,  in  combination  and  free  solid. 

FeO,  ferrous  oxide  —  only  in  combination  in  solids- 

MgO,  magnesia,  in  combination  and  free,  solid.* 

CaO,  lime  —  only  in  combination  in  solids. 

Na,,O,  soda  —  only  in  combination  in  solids. 

K2O,  potassa  —  only  in  combination  in  solids. 

The  above  table  is  given  in  order  of  decreasing  acidity 
and  increasing  basicity  from  top  to  bottom.  To  this  list 
we  should  also  add  water,  H2O,  which  occurs  free  in  the 
gaseous,  solid  and  liquid  states,  and  in  combination. 

Since  we  are  considering  rocks  it  is  evident  that  of 
these  oxides  and  their  combinations  we  need  to  regard 
only  those  which  form  solids.  They  are  then  the  com- 
pounds of  silicic  and  carbonic  acid  or  silicates  and  car- 
bonates and  the  oxides  of  silica,  alumina  and  iron. 

Ice,  the  solid  form  of  water,  may  also  be  regarded  as  a  rock, 
but  as  such,  needs  no  further  consideration  in  this  work.  Com- 
binations of  the  oxides  of  aluminium,  iron  and  magnesium  and 
of  the  silicates  with  water  also  occur.  Combinations  with  sulphur 
as  sulphides  and  sulphates  and  of  phosphorus  as  phosphates  and 
of  chlorine  as  chlorides  are  at  times  of  local  importance  though 
never  having  the  general  interest  of  those  mentioned  above. 
They  receive  attention  in  their  appropriate  places. 

*  The  solid  MgO  is  the  mineral  periclase,  excessively  rare,  and  of  no  petro 
logic  importance. 


PART     II. 

ROCK  MINERAI& 


CHAPTER  III. 
IMPORTANT  PROPERTIES  OF  MINERALS. 

SINCE  all  rocks,  with  the  exception  of  a  few  glassy  ones 
of  igneous  origin,  are  composed  of  minerals  it  is  of  first 
importance  in  their  study  and  determination  that  a  good 
knowledge  of  the  important  rock-making  minerals,  of 
their  obvious  characters  and  properties,  should  be  had. 
This  is  so  indispensable,  that  before  taking  up  the  rocks 
themselves,  the  following  part  of  this  work  is  devoted: 
first,  to  a  general  account  of  those  properties  of  minerals 
which  are  of  value  in  megascopic  determination;  second, 
in  a  succeeding  chapter,  to  a  description  of  the  minerals 
individually;  and  third,  to  methods  for  their  determination. 

Minerals  defined.  A  mineral  is  defined  as  any  inorganic 
substance  occurring  in  nature  which  possesses  a  definite 
chemical  composition.  To  this,  for  petrographic  purposes 
we  should  add,  that  it  is  also  a  solid  and  usually  it  has  a 
definite  crystalline  structure.  The  word  is  also  used  in  two 
ways  with  different  meanings:  in  one,  which  may  be  termed 
the  abstract  chemical  way,  we  refer  to  a  compound  having 
a  certain  composition,  as  in  speaking  of  calcite  we  mean 
the  compound  CaCO.3,  carbonate  of  lime;  in  the  other 
when  we  speak  of  the  minerals  of  a  rock  we  refer  to  the 
actual  crystal  grains,  the  minerals  as  distinct  entities  or 
bodies  which  compose  that  rock. 

Crystals  defined.  Most  chemical  compounds  when 
their  molecules  are  free  to  arrange  themselves  in  space  and 
the  conditions  are  proper  for  them  to  assume  solid  form, 

21 


22        ROCKS  AND  ROCK  MINERALS 

as  for  example  when  they  solidify  from  solutions,  appear 
in  crystals.  That  is,  the  molecules  arrange  themselves  in 
a  definite  geometric  system,  characteristic  of  that  com- 
pound, and  governed  by  mathematical  laws,  which  give 
the  solid  a  definite  internal  structure  and  an  outward 
form  bounded  by  planes  which  are  always  placed  at 
certain  angles  to  one  another.  Thus  minerals  crystallize 
in  cubes,  octahedrons,  prisms,  etc.  The  conditions  in 
rock  formation  are  sometimes  such  that  a  mineral  can 
assume  outward  crystal  form,  and  it  is  then  bounded  by 
distinct  planes;  more  commonly,  however,  the  growing 
crystals  interfere  with  one  another  and  have  no  distinct 
form,  or,  as  in  the  sedimentary  rocks,  they  are  fragments 
only  of  former  crystals,  or  their  plane  surfaces  have  been 
worn  off  by  attrition.  Since,  however,  they  possess  the 
inward  characteristic  structure  we  still  call  such  bodies 
crystals,  though  lacking  the  outward  form.  Thus,  when 
we  speak  of  the  crystals  or  crystal  grains  composing  a 
rock,  we  do  not  necessarily  imply  that  these  have  plane 
surfaces  which  give  them  geometric  forms.  Such  grains 
are  sometimes  called  anhedrons  (from  the  Greek,  meaning, 
without  planes). 

List  of  Properties.  The  chief  properties  of  the  rock- 
making  minerals  by  which  they  may  be  known  are  their 
crystal  form,  color,  cleavage  and  associations;  these  are 
perceived  by  the  eye,  and  in  addition  we  have  their  hard- 
ness, specific  gravity,  and  their  behavior  before  the  blow- 
pipe and  with  chemical  reagents,  properties  which  demand 
some  form  of  testing  with  apparatus. 

Crystal  form.  The  mineral  grains  which  compose 
rocks  do  not,  as  a  rule,  possess  good  outward  crystal  form 
as  mentioned  in  a  preceding  paragraph.  The  reason  for 
this  is  that  in  the  igneous  and  metamorphic  rocks,  the 
growing  minerals  interfere  with  one  another's  develop- 
ment, and  thus,  while  they  may  roughly  approximate  to 
a  certain  general  shape  the  mineral  has  endeavored  to 
assume,  the  outer  surface  is  not  composed  of  smooth 


IMPORTANT  PROPERTIES  OF  MINERALS  23 

definite  planes;  while  in  the  sedimentary  rocks  the  grains 
are  either  broken  fragments  or  rounded  by  rolling  and 
grinding.  It  may  happen,  however,  that  in  a  liquid 
molten  mass  when  crystallization  begins,  one  or  more 
kinds  of  minerals  may  commence  growing  in  crystals 
scattered  through  it  and  complete  their  period  of  growth 
before  the  others  which  will  compose  the  general  mass  of 
the  rock  have  commenced.  In  this  case  they  have  not 
been  interfered  with,  and  they  may  exhibit  good  outward 
crystal  forms  bounded  by  distinct  planes.  This  is  well 
shown  in  those  kinds  of  igneous  rocks  which  are  described 
elsewhere  as  porphyries.  Likewise  in  the  metamorphic 
rocks,  certain  minerals  often  appear  in  such  well-bounded, 
distinct  crystals,  as  to  indicate  that  they  are  of  later  origin, 
and  although  formed  by  molecular  rearrangement  of 
materials  in  a  solid  or  somewhat  plastic  mass  the  con- 
ditions were  such  that  they  were  not  interfered  with 
during  their  period  of  growth.  This  is  illustrated  by  the 
excellent  crystals  of  garnet,  often  seen  in  the  rock  known 
as  mica  schist. 

Thus  in  general  the  outward  crystal  form  or  shape  of 
minerals  in  rocks  is  wanting  and  cannot  be  used  as  a 
means  for  determining  them,  but  in  many  special  cases  it 
may  be  well  developed  in  rock-making  minerals  and  it 
can  then  be  very  useful.  The  shapes  in  which  each 
mineral  is  most  apt  to  occur  are  described  under  the 
heading  of  that  mineral  in  the  descriptive  part. 

Color.  The  color  of  minerals,  when  used  with  proper 
precautions,  is  also  a  very  useful  property  for  helping  to 
distinguish  them.  The  color  of  minerals  is  dependent 
upon  their  chemical  composition,  in  which  case  it  may  be 
said  to  be  inherent,  or  it  may  be  due  to  some  foreign  sub- 
stance distributed  through  them  and  acting  as  a  pigment, 
and  their  color  may  then  be  termed  exotic.  It  is  because 
the  color  of  the  mineral  grains  of  rocks  is  frequently  exotic 
that  precaution  must  be  used  in  employing  it  as  a  means 
of  discrimination. 


24  ROCKS  AND  ROCK  MINERALS 

In  regard  to  inherent  colors,  neither  silica  nor  carbonic 
acid  in  combination  as  silicates  and  carbonates  has  any 
capability  for  producing  color,  and  so  far  as  they  are  con- 
cerned, such  compounds  would  be  colorless,  or,  as  will  be 
presently  explained,  white.  So,  silica  alone,  as  quartz,  is 
naturally  colorless.  The  same  is  also  true  of  the  metallic 
oxides  alumina,  lime,  magnesia,  and  the  alkalies  soda  and 
potash.  Thus,  carbonate  of  lime  or  calcite,  carbonate  of 
lime  and  magnesia  or  dolomite,  oxide  of  alumina  or  corun- 
dum, silicates  of  lime  and  magnesia,  silicates  of  alumina 
and  the  alkalies  or  feldspars  are  all  inherently  colorless 
minerals.  The  metallic  oxides  which  chiefly  influence 
the  color  of  rock  minerals  are  those  of  iron,  chromium 
and  manganese,  and  the  only  one  of  these  which  is  of 
wide  petrographic  importance  is  iron,  especially  iron  as 
ferric  oxide.  The  minerals  containing  iron  as  a  prominent 
component  are  dark  green,  dark  brown  or  black,  and 
these  colors  may  ordinarily  be  regarded  as  indicative  of 
this  metal. 

With  respect  to  the  exotic  colors  which  minerals  fre- 
quently exhibit,  this  may  be  due  to  one  of  two  causes. 
It  may  happen  that  a  minute  amount  of  some  compound 
of  an  intensely  colorative  character  is  present  in  chemical 
combination.  Thus  a  minute  amount  of  manganese 
oxide  in  quartz  is  supposed  to  produce  the  amethyst 
color,  traces  of  chromic  oxide  sometimes  color  silicates 
green,  and  probably  copper  does  also.  Or  the  color  may 
be  due  to  a  vast  number  of  minute  bodies  dispersed 
through  the  crystal  as  inclusions.  These  minute  specks 
may  have  a  distinct  color  of  their  own  and  thus  act  as  a 
pigment,  as  when,  for  example,  quartz  is  colored  dark  red 
by  a  reddish  dust  of  ferric  oxide  particles  in  it;  or  the 
inclusions  may  be  so  arranged  in  regular  systems  as  to 
act  refractively  upon  light,  breaking  it  up  and  producing 
a  play  of  prismatic  colors,  or  opalescence  in  the  substance. 
Usually  in  the  latter  case,  one  color  predominates  and 
gives  its  character  to  the  mineral.  A  good  example  of 


IMPORTANT  PROPERTIES  OF  MINERALS  25 

this  is  seen  in  the  variety  of  feldspar  called  labradorite,  a 
constituent  of  the  rocks  called  anorthosite  and  gabbro, 
which  often  shows  a  fine  play  of  colors,  a  rich  dark  blue 
being  usually  predominant. 

The  white  color  which  so  many  rock-making  minerals 
exhibit  may  be  due  to  minute  inclusions,  as  when  feld- 
spars are  sometimes,  through  alteration,  filled  with  scales 
of  kaolin  or  white  mica,  but  more  commonly  it  is  due  to 
the  reflection  of  light  from  the  surfaces  of  innumerable 
microscopic  cracks  and  crevices  which  everywhere  per- 
meate the  mineral  substance.  In  such  cases  the  material 
is  really  colorless  and  transparent.  The  effect  is  the  same 
as  if  a  piece  of  colorless  glass  should  be  ground  to  powder, 
y/hich  would  of  course  be  white.  Hence  white  minerals 
are  not  regarded  as  possessing  any  color,  and  they  are 
often  free  from  such  cracks  and  are  then  colorless  and 
transparent.  Good  examples  of  this  are  seen  in  such 
common  minerals  as  quartz,  calcite  and  feldspars.  As 
explained  under  cleavage,  these  cracks  in  feldspar  are 
sometimes  so  regularly  arranged  as  to  produce  a  play  of 
colors,  giving  the  mineral  an  opalescence  or  pearly  luster 
with  a  distinctly  predominating  color  tone  like  that 
mentioned  above  as  produced  by  inclusions. 

Streak.  In  addition  to  that  color  which  minerals  show 
in  the  solid  form,  there  is  another  way  in  which  this 
property  may  be  often  usefully  employed  in  determining 
them.  This  is  the  color  which  the  substance  presents 
when  reduced  to  a  state  of  powder.  The  powder  may  be 
obtained  by  grinding  a  small  fragment  in  a  mortar,  but 
it  is  more  easily  produced  by  scratching  a  sharp  point 
of  the  mineral  across  a  plate  of  unglazed  porcelain;  the 
color  of  the  resultant  streak  is  of  course  that  of  the 
powdered  mineral.  While  any  piece  of  unglazed  porce- 
lain will  answer  fairly  well,  pmall  plates  are  specially 
made  for  this  purpose  and  sold  by  dealers  in  chemical 
apparatus. 


26         ROCKS  AND  ROCK  MINERALS 

The  color  shown  by  minerals  in  the  powdered  state  is  usually 
much  lighter  than  that  which  they  exhibit  in  the  mass  and  sometimes 
very  different.  It  is  most  useful  in  helping  to  discriminate  the 
dark  colored  minerals,  especially  the  metallic  oxides  and  sulphides 
of  the  heavy  metals  used  as  ores,  and  hence  its  application  with  the 
light  colored  silicates  and  carbonates  that  chiefly  form  the  rock- 
making  minerals  is  much  more  limited  and  of  lesser  value.  In  the 
case  of  these  minerals  it  is  sometimes  useful  in  distinguishing  exotic 
from  natural  colors;  for  the  color  of  the  streak  is  generally  that  of 
the  mineral  substance  itself,  and  the  pigment  or  other  impurities 
which  produce  an  exotic  color  must  be  present  in  very  large  amount 
to  exert  a  definite  influence.  Thus  calcite  is  colorless  or  white,  but 
sometimes  yellow,  brown  or  red,  but  the  streak  of  all  these  colors 
is  white  or  barely  tinted  except  in  unusual  instances.  The  feld- 
spars are  normally  white  or  colorless,  but  in  some  rocks,  such  as 
anorthosite,  they  sometimes  are  black  and  at  first  glance  might  be 
mistaken  for  an  iron-bearing  mineral;  the  streak,  however,  is  white 
and  helps  to  show  their  true  character. 

In  the  field,  the  bruised  surface  of  the  rock,  where  struck  by  the 
hammer,  often  shows  the  powdered  minerals,  giving  in  a  rough 
manner  the  color  of  the  streak;  or  a  bit  of  the  substance  may  be 
ground  between  two  hammer  surfaces  and  the  powder  rubbed  on 
white  paper. 

Cleavage.  When  mineral  bodies  possess  crystalline 
structure,  it  frequently  happens  that  the  arrangement  of 
the  physical  molecules  composing  them  is  such,  that  the 
force  of  cohesion  among  them  is  less  in  some  particular 
direction  or  directions  than  in  others.  Along  such 
directions,  if  suitable  means  be  employed,  such  as  placing 
the  edge  of  a  knife  upon  the  mineral  and  striking  with  a 
hammer,  the  body  will  tend  to  split  or  cleave.  The  degree 
of  perfection  with  which  minerals  possess  this  property 
is  very  variable;  some,  like  mica,  which  is  used  for  stove 
windows  and  lamp  chimneys,  are  capable  of  being  almost 
indefinitely  split  into  thin  leaves ;  others,  like  the  feldspars, 
have  a  good  cleavage;  while  some,  like  quartz,  have  no 
apparent  cleavage.  When  the  cleavage  is  very  good  the 
new  surfaces  are  smooth  and  shining  like  the  original  ones 
of  a  crystal  and  it  is  termed  perfect.  This  property, 
being  then  so  distinctive,  is  a  most  useful  one  in  helping 


IMPORTANT  PROPERTIES  OF  MINERALS  27 

to  determine  minerals,  especially  in  rocks  where  the 
mineral  grains  on  the  surface,  broken  by  the  hammer,  if 
they  possess  it,  everywhere  show  shining  cleavage  faces. 
It  must  not  be  imagined  that  the  directions  of  cleavage 
occur  at  random  in  a  mineral;  on  the  contrary,  they  always 
bear  a  definite  relation  to  the  special  crystal  form  that 
characterizes  a  particular  mineral.  If  the  latter  has  two 
directions  in  which  it  may  be  cleaved,  like  feldspar,  for 
example,  the  angle  between  the  two  surfaces  is,  for  a 
feldspar  of  a  certain  definite  chemical  composition,  always 
the  same.  Some  minerals,  like  mica,  have  only  one 
direction  in  which  there  is  good  cleavage;  others  have  two 
directions,  and  sometimes  the  two  are  exactly  alike  and 
sometimes  unlike,  one  being  better  than  the  other;  again 
there  may  be  three  directions  in  which  cleavage  can  be 
produced,  all  alike  as  in  calcite  or  unlike  as  in  barite 
(heavy  spar,  BaS04),  or  there  may  even  be  four  or  more. 
Whether  the  cleavages  are  alike  or  unlike,  when  there  is 
more  than  one,  depends  not  only  on  their  direction  in  the 
crystal,  but  also  on  the  geometric  form  or  system  of 
crystallization  the  latter  exhibits.  A  description  of  these 
relations  would  involve  too  much  of  the  principles  of 
crystallography  for  discussion  in  this  place,  but  the 
following  will  be  helpful  in  understanding  certain  terms 
frequently  used. 

A.  Good  cleavage  in  one  direction  only:   the  mineral 
grains  in  the  rock  in  this  case  are  apt  to  be  developed  in 
tables,  folia,  or  scales,  whose  surface  is  parallel  to  the 
cleavage.     This  is  well  shown  in  such  minerals  as  the 
micas  and  chlorite. 

B.  Good   cleavage  in  two  directions  and  both  alike: 
the  minerals  are  apt  to  be  developed  in  elongated  forms 
parallel  to  the  cleavage,  and  the  latter  is  spoken  of  as  being 
prismatic.     This  is  shown  by  the  minerals  hornblende 
and  pyroxene.     If  the  two  cleavages  are  not  exactly 
alike,  the  mineral  still  is  often  elongated  in  the  direction 
of  the  edge  produced  by  the  meeting  of  the  two  cleavage 


28  ROCKS  AND  ROCK  MINERALS 

planes.  It  may  be  sometimes  columnar  and  sometimes 
tabular  parallel  to  the  better  cleavage.  The  feldspars, 
which  form  the  free  developed  crystals  in  porphyry,  often 
show  such  relations. 

C.  Good  cleavage  in  three  directions  alike:  if  the  three 
planes  are  at  right  angles  to  each  other  the  mineral  will 
break  up  into  cubes  and  the  cleavage  is  cubic  or  apparently 
so;  if  they  are  at  some  other  angle,  rhombs  will  be  produced 
and  the  cleavage  is  called  rhombohedral.  Cubic  cleavage 
is  well  shown  by  galena,  PbS,  the  common  ore  of  lead  and 
by  rock  salt;  it  is  not  exhibited  by  any  common  rock- 
making  mineral.  Rhombohedral  cleavage  is  character- 
istic of  the  common  rock-making  carbonates,  calcite, 
CaCOs,  and  dolomite  (MgCa)CO3.  Three  unlike  directions 
have  the  same  practical  effect  as  two  unlike,  and  four 
directions  are  not  of  importance  in  megascopic  petro- 
graphy, as  no  common  rock-making  mineral  exhibits 
them. 

If  a  rock  with  component  mineral  grains  sufficiently  coarse  so 
that  they  can  be  readily  studied  by  the  pocket-lens,  the  size  of  peas, 
for  example,  be  carefully  examined,  it  will  be  found  that  almost 
without  exception,  where  a  mineral  shows  a  cleavage  face,  'it  will  be 
full  of  minute  cracks  and  fissures.  These  cracks  are  paral'el  some- 
times to  one  cleavage  and  sometimes  to  all  the  cleavage  directions 
the  mineral  has.  In  addition  to  the  cleavage  cracks  there  are  also 
irregular  lines  of  fracture  which  do  not  correspond  to  any  definite 
direction.  Commonly,  the  mineral  grains  of  rocks  contain  not  only 
these  large  cleavage  cracks  and  irregular  fractures  which  can  be 
perceived  with  the  eye  or  with  the  aid  of  the  lens,  but,  in  addition, 
they  are  everywhere  rifted  by  similar  ones  so  minute  that  they  can 
only  be  detected  in  thin  sections  of  rocks  under  high  powers  of  the 
microscope.  It  is  the  reflection  of  light  from  these  minute  micro- 
scopic cracks  that  renders  so  many  minerals  opaque  and  white  in 
color  that  would  otherwise  be  colorless  and  transparent.  These 
cracks  and  fissures  have  been  produced  in  the  rocks  by  the  various 
forces  to  which  they  have  been  subjected;  sometimes  they  are  due 
to  the  contraction  following  a  heated  stage  as  in  metamorphic  and 
igneous  rocks,  and  sometimes  and  more  generally  to  the  intense 
pressures  and  strains  to  which  the  rocks  of  the  earth's  crust  are  and 
have  been  subjected.  Minute  as  the  rifts  in  the  mineral  grains  are, 


IMPORTANT  PROPERTIES  OF  MINERALS 


29 


they  are  of  great  importance  in  geologic  processes,  for  by  means  of 
them,  and  drawn  by  capillary  action  with  great  force,  water  con- 
taining CO  2  in  solution  penetrates  not  only  the  rocks  but  the  indi- 
vidual grains  as  well,  to  their  very  interiors,  and  alters  and  changes 
them  into  other  minerals  and  the  rocks  into  soil. 

Fracture.  The  appearance  of  the  breakage  of  minerals 
in  directions  which  are  not  those  of  cleavage  or  in  cases 
where  the  mineral  does  not 
possess  cleavage  is  called  its 
fracture.  If  the  mineral  is 
fibrous  in  structure,  the 
fracture  may  be  fibrous;  or 
it  may  be  rough  and  un- 
even or  hackly;  if  .  the 
mineral  is  dense,  compact 
and  homogeneous  it  will  be 
conchoidal,  that  is,  it  will 
present  a  sort  of  shelly 
appearance  such  as  is  shown 
on  surfaces  of  broken  glass 
which  recall  the  inside  or 
outside  of  a  clam  shell. 
Rocks  which  are  extremely  dense  and  homogeneous,  like 
some  flints,  or  glassy  lavas  or  fine-grained  compact  ones, 
have  also  a  conchoidal  fracture  more  or  less  pronounced. 
Quartz  is  the  most  common  mineral  which  gives  a  good 
example  of  conchoidal  fracture. 

Associations  of  Minerals.  The  facts  that  certain  kinds 
of  minerals  are  apt  to  be  found  together  in  the  same  kind 
of  rock  and  that  the  presence  of  one  mineral  excludes 
the  presence  of  some  other  mineral  are  of  great  value  in 
petrography  but  of  much  greater  use  in  microscopic  work, 
where  the  distinguishing  characters  of  minerals  are  easily 
made  out,  than  in  field  determinations.  Even  in  mega- 
scopic petrography,  however,  these  facts  are  at  times  of 
practical  use;  thus  the  fact  that  the  two  minerals,  quartz 
and  nephelite,  cannot  occur  naturally  together  as  rock- 


Fig.  4.  Conchoidal  fracture  in  obsidian 
volcanic  glass. 


30  ROCKS  AND  ROCK  MINERALS 

making  components  is  of  value  in  discriminating  between 
certain  rocks.  The  various  relations  of  this  kind  that  are 
of  importance  will  be  mentioned  in  their  appropriate  places. 
Hardness.  This  property  is  of  great  value  in  helping 
to  make  determinations  of  minerals,  and  it  is  likewise  very 
useful  in  the  field  in  making  rough  tests  of  rocks.  The 
hardness  of  minerals  is  determined  by  comparing  them 
with  the  following  scale: 

Scale  of  Hardness 

1.  Talc.  6.  Feldspar. 

2.  Gypsum.  7.  Quartz. 

3.  Calcite.  8.  Topaz. 

4.  Fluorite.  9.  Corundum. 

5.  Apatite.  10.  Diamond. 

This  means  that  each  mineral,  using  a  sharp  point,  will 
scratch  smooth  surfaces  of  all  the  minerals  in  the  list  above 
it  but  of  none  below  it.  If,  for  example,  a  fragment  of  an 
unknown  mineral  is  found  to  scratch  calcite  its  hardness 
is  greater  than  3;  if  it  will  not  scratch  fluorite,  but,  on  the 
contrary,  is  scratched  by  it,  its  hardness  is  not  so  great 
as  4,  but  must  be  between  3  and  4  or  approximately  3^. 

The  point  of  a  pocket-knife  blade  as  ordinarily  tempered 
with  a  hardness  of  a  little  over  5  and  pieces  of  common 
window  glass  with  hardness  of  about  5^  are  very  useful 
for  testing  the  hardness  of  common  minerals  and  of  the 
rocks  made  up  of  them.  A  common  brass  pin  point  is  a 
little  over  3  and  will  scratch  calcite;  the  finger  nail  is  a 
little  over  2  and  will  scratch  gypsum. 

Specific  Gravity.  The  specific  gravity  of  a  substance 
is  its  density  compared  with  water  or  the  number  of  times 
heavier  a  given  volume  of  the  substance  is  than  an  equal 
volume  of  water.  It  is  obtained  by  weighing  a  piece  of 
the  mineral  or  rock  in  air  and  then  in  water;  the  difference 


IMPORTANT  PROPERTIES  OF  MINERALS 


31 


between  the  two  is  equal  to  the  weight  of  an  equal  volume 
of  water  (the  volume  displaced)  and  we  have 


wt.  in  air 


wt.  in  air  —  wt.  in  water 


=  Sp.  Gr, 


The  operation  may  be  carried  out  with  one  of  the  special  forms 
of  apparatus  devised  for  determining  specific  gravity  and  described 
in  the  manuals  of  determinative  mineralogy,  or  it  may  be  done  with 
a  chemical,  an  assay  or  a  jeweler's  balance.  It  is  first  weighed  in 
the  pan  and  then  suspended  from  it  by  a  hair  and  weighed  in  water. 


SPECIFIC   GRAVITIES    OF   ROCK    MINERALS. 

ARRANGED    IN    DESCENDING   ORDER. 


5.2 

Magnetite. 

2.86 

Muscovite. 

5.2 

Hematite 

2.85 

Dolomite. 

4.9-5.1 

Pyrite. 

2.80 

Talc. 

4.7-5.1 

Ilmenite. 

2.75 

Anorthite. 

3.95-4.1 

Corundum. 

2.73 

Labradori  te. 

3.6-4.0 

Limonite. 

2.72 

Calcite. 

3.5-4.2 

Garnet. 

2.65-2.75 

Chlorite. 

3.75 

Staurolite. 

2.66 

Quartz. 

3.5 

Topaz. 

2.62 

Albite. 

3.56-3.66 

Cyanite. 

2.6 

Kaolin. 

3.4 

Vesuvianite. 

2.5-2.65 

Serpentine. 

3.2-3.5 

Pyroxene. 

2.57 

Orthoclase. 

3.27-3.37 

Olivine. 

2.55-2.65 

Nephelite. 

3.25-3.45 

Epidote. 

2.45-2.50 

Leucite. 

3.0-3.4 

Amphibole. 

2.32 

Gypcum. 

3.16-3.2 

Andalusite. 

2.27 

Analcite. 

3.1-3.2 

Chondrodite. 

2.25 

Natrolite. 

3.15 

Apatite. 

2.15-2.30 

Sodalite. 

3.0-3.15 

Tourmaline. 

2.15-2.2 

Heulandite. 

2.95 

Anhydrite. 

2.1-2.2 

Stilbite. 

2.8-3.1 

Biotite. 

2.1-2.2 

Opal. 

A  piece  about  one-half  inch  in  diameter  is  convenient  both  for 
minerals  and  rocks,  but  in  the  case  of  minerals  it  is  frequently 
necessary  to  select  a  fragment  smaller  than  this  to  obtain  pure 
homogeneous  material,  without  which  it  is  perhaps  needless  to  say 
the  determination  is  of  little  value.  Adherent  air  bubbles  and  air 
in  cracks  are  best  gotten  rid  of  by  boiling  the  fragment  in  water  and 
then  allowing  it  to  cool  before  weighing.  If  the  mineral  has  an 


32  ROCKS  AND  ROCK  MINERALS 

invariable  chemical  composition  and  crystal  form,  as  for  example, 
quartz  (SiO2),  calcite  (CaCO3),  etc.,  the  specific  gravity  is  an  invari- 
able quantity,  and  departures  from  it  must  be  due  to  the  presence 
of  impurities.  Many  minerals,  however,  while  they  retain  the  same 
crystal  form,  vary  considerably  in  chemical  composition  in  that  one 
metallic  oxide  may  be  more  or  less  replaced  by  another  similar  oxide 
or  oxides.  Thus  we  find  minerals  which  at  one  end  of  a  series  con- 
tain magnesia,  MgO,  and  at  the  other  end  ferrous  oxide,  FeO,  and 
between  these  extremes  all  degrees  of  mixtures  of  these  two  oxides. 
In  accordance  with  such  variations  the  specific  gravity  of  the  mineral 
varies.  The  pyroxenes,  amphiboles,  garnets,  olivines,  etc.,  are 
examples  of  this,  and  it  accounts  for  most  of  the  variations  in  specific 
gravity  which  may  be  observed  in  the  annexed  table. 

Blowpipe  Reactions.  The  rock-making  minerals,  which 
are  chiefly  carbonates  and  silicates,  do  not  as  a  rule 
exhibit  before  the  blowpipe  very  characteristic  reactions 
by  which  they  may  be  readily  determined,  as  do  so  many 
of  the  ores,  the  oxides  and  sulphides  of  the  heavy  metals. 
Still,  however,  the  relative  degree  of  fusibility  shown  by 
thin  splinters,  the  coloration  of  the  flame  and  the  characters 
of  the  melted  bead  which  may  result  are  properties  which 
may  be  of  great  service  in  helping  to  determine  these 
minerals,  and  so  far  as  they  have  value  in  this  direction 
they  are  mentioned  in  the  description  of  the  minerals. 
If  instruction  in  the  use  of  the  blowpipe  is  desired  it  should 
be  sought  in  one  of  the  manuals  devoted  to  that  purpose. 

Chemical  Reactions  with  Reagents.  Certain  qualitative 
chemical  tests  which  can  generally  be  made  with  a  few 
reagents  and  simple  apparatus  are  of  great  service  in 
mineral  determination  and  in  aiding  to  classify  rocks. 
In  Chapter  V,  in  which  the  methods  for  the  identification 
of  minerals  are  given,  these  tests  and  the  proper  ways  of 
making  them  are  fully  described. 


CHAPTER  IV. 

THE  ROCK-MAKING  MINERALS. 
SEC.  1.     Primary  Anhydrous  Silicates  and  Oxides. 

THESE  minerals  from  the  geological  standpoint  are  th^ 
most  important  in  forming  rocks.  They  are  the  most 
abundant  and  the  most  widely  diffused.  They  are  the 
chief  minerals  which  are  formed  by  the  cooling  and 
crystallization  of  the  molten  fluids  of  the  earth's  interior, 
and  hence  they  are  the  main  components  of  the  igneous 
rocks.  The  greater  part  of  the  metamorphic  rocks  are 
also  made  up  of  them,  and  in  the  sedimentary  beds  they 
are  also  important  constituents  in  many  cases. 

It  is  difficult  to  draw  a  sharp  line  between  the  absolutely 
anhydrous  minerals  and  those  containing  considerable 
quantities  of  combined  water.  Thus,  most  hornblendes, 
micas  and  epidotes  contain  small  amounts  of  hydroxyl 
and  yet  are  ordinarily  considered  as  anhydrous,  compared, 
for  instance,  with  kaolin,  serpentine  and  chlorite.  In 
the  same  way  feldspar,  hornblende  and  pyroxene  are 
thought  of  as  primary  minerals  although  we  know  that  in 
some  cases  they  are  of  secondary  origin,  that  is,  they  have 
been  formed  at  the  expense  of  previously  existent  minerals. 
The  grouping  as  given  is  largely  a  matter  of  convenience; 
it  includes  those  which  are  always  anhydrous  and  always 
primary  and  which  thus  give  a  certain  distinctive  charac- 
ter to  the  group,  which  it  is  well  to  enforce,  but  it  also 
includes  many  which  are  at  times  secondary  and  some 
which  are  hydrous,  because  on  account  of  their  mineralogic 
positions  and  affinities  it  is  more  convenient  and  natural 
to  consider  these  minerals  in  this  connection. 

In  the  following  section  only  such  silicates  and  oxides 
are  treated  as  are  both  hydrous  and  secondary. 

33 


34  ROCKS  AND  ROCK  MINERALS 

a.     Silicates. 

The  silicates  are  salts  of  various  silicic  acids,  in  which 
the  hydrogen  atoms  have  been  replaced  by  various  metals 
or  radicals  composed  of  metals  in  combination  with 
oxygen,  hydroxyl,  fluorine,  etc.  The  three  important 
silicic  acids  which  in  this  group  form  rock  minerals  are 
poly  silicic  acid,  EUSisOg;  metasilicic  acid,  H2SiO3,  and 
orthosilicic  acid,  EUSiO^  The  list  of  those  treated  as  of 
importance  on  account  of  the  functions  which  they  have 
as  rock-making  minerals  includes  the  feldspar,  felds- 
pathoid,  mica,  pyroxene,  amphibole,  olivine,  garnet, 
tourmaline  and  epidote  groups,  and  a  few  other  less 
common  ones. 

FELDSPARS. 

The  term  feldspar  is  not  the  name  of  a  single  mineral 
of  a  definite  chemical  composition  like  quartz,  Si02,  but 
is  the  designation  of  a  group  of  minerals  which  have  a 
general  similarity  in  chemical  and  physical  properties. 
They  are  indeed  so  much  alike  in  general  characters  and 
appearance  that  in  determining  rocks  by  megascopic 
features  they  cannot  be  told  apart  except  in  special  cases, 
and  it  is,  therefore,  best  to  treat  them  as  a  group,  and  at 
the  same  time  mention  those  characters  by  which,  when 
possible,  they  may  be  distinguished. 

The  rock-making  feldspars  are  composed  of  three  kinds 
and  their  mixtures  as  follows : 

a.  Orthoclase,  KAlSisOg,  silicate  of  potash  and  alumina; 

b.  Albite,  NaAlSisOg,  silicate  of  soda  and  alumina; 

c.  Anorthite,  CaAl2Si20g,  silicate  of  lime  and  alumina; 
Alkalic  feldspar,  (KNa)AlSi3Og,  mixtures  of  a  and  6; 
Plagioclase  feldspar,  (NaAlSiaOg)*  +  (CaAl2Si2O8)y, 

mixtures  of  b  and  c. 

The  simple  feldspars  are  mostly  confined  to  the  crystals 
found  in  veins,  druses,  etc.;  they  sometimes  occur  as  the 


ROCK-MAKING  MINERALS 


35 


component  grains  of  rocks,  but  are  comparatively  rare; 
in  the  great  majority  of  cases  the  feldspars  are  either 
mixtures  of  orthoclase  and  albite  in  varying  proportions 
but  usually  with  a  considerable  excess  of  the  potash 
compound  and  are  then  called  alkalic  feldspar,  or  they  are 
mixtures  of  albite  and  anorthite  and  are  then  known  as 
soda-lime  feldspar  or  plagioclase.  All  transitions  from 
pure  albite  to  pure  anorthite  occur,  and  the  series  has  been 
divided  into  groups  according  to  the  different  proportions 
of  the  soda  and  lime  molecules;  one  of  the  most  important 
of  these  is  called  labradorite  in  which  there  are  about 
equal  amounts  of  the  two  kinds. 

Mixtures  of  a  and  c,  of  the  potash  and  lime  feldspars,  have  been 
found  to  occur  but  are  so  rare  that  for  practical  purposes  they  may 
be  neglected. 

Form.  Orthoclase  is  monoclinic  in  symmetry,  and  when 
in  distinct  well-made  crystals  it  commonly  takes  the 


Fig.  5. 


Fig.  e. 


Fig.  7. 


forms  shown  in  the  accompanying  figures.  Sometimes 
the  crystals  are  stout  and  thick  in  their  habit  or  appear- 
ance as  in  Fig.  5,  sometimes  they  are  thin  and  tabular 
parallel  to  the  face  6  as  in  Fig.  6,  and  again  they  may 
be  rather  long  and  columnar  as  in  Fig.  7.  In  or- 
thoclase the  face  c  is  always  at  right  angles  with  the 
face  6.  In  albite  and  anorthite,  whose  crystallization  is 


36         ROCKS  AND  ROCK  MINERALS 

triclinic,  these  faces  c  and  6  are  not  at  right  angles  but  ara 
slightly  oblique;  this  is  also  true  for  all  of  their  mixtures 
or  the  plagioclase  group  in  general.  Some  mixtures  of 
orthoclase  and  albite,  as  well  as  certain  varieties  of  the 
pure  potash  compound  KAlSiaOg  called  microcline,  are 
also  slightly  oblique,  but  in  all  these  cases  mentioned  the 
amount  of  departure  from  a  right  angle  is  only  a  few 
degrees  which,  even  under  favorable  conditions,  can 
scarcely  be  perceived  by  the  eye  and  must  be  measured 
by  a  goniometer  to  be  appreciated.  It  cannot,  therefore, 
under  ordinary  circumstances,  be  used  as  a  means  of 
discrimination  between  the  alkalic  and  plagioclase  feld- 
spars. The  forms  of  the  crystals  in  which  the  plagioclase 
feldspars  appear  in  rocks  when  they  have  the  opportunity 
to  crystallize  freely  are  similar  to  those  mentioned  above 
for  orthoclase  in  Figs.  5-7. 

It  is  only  in  the  phenocrysts  of  porphyritic  igneous 
rocks  and  in  the  miarolitic  druses  of  the  massive  igneous 
ones  that  these  minerals  have  an  opportunity  to  assume 
the  free  crystal  forms  described;  in  ordinary  cases  their 
crystallization  is  interfered  with  by  other  minerals  or  by 
other  crystals  of  feldspar  and  they  are  thus  seen  in  shape- 
less masses  or  grains.  Nevertheless  there  is  a  tendency 
to  assume  these  forms,  and  in  some  rocks,  such,  for  instance, 
as  the  syenites,  which  are  mainly  composed  of  feldspar, 
it  may  be  observed  that  they  have  more  or  less  perfectly 
the  shape  of  flat  tables  or  rude  laths  as  they  approximate 
to  Figs.  6  or  7. 

Twinning.  Crystals  frequently  appear  compound,  as 
if  cut  through  parallel  to  some  prominent  plane  on  them 
and  one  of  the  halves  revolved  180  degrees,  usually  on  an 
axis  perpendicular  to  the  plane  of  division  which  is  called 
the  twinning  plane,  and  the  two  parts  grown  together. 
Such  an  arrangement  is  called  a  twin  crystal.  Feldspars 
very  commonly  occur  in  twin  crystals,  one  of  the  most 
frequent  arrangements  being  that  illustrated  in  Fig.  8 
and  known  as  the  Carlsbad  twinning  from  the  town  of  that 


ROCK-MAKING    MINERALS 


37 


Fig.  8 


Fig  9 


name  in  Bohemia  where  excellent  examples  have  been 
found.  It  is  as  if  a  crystal  like  that  shown  in  Fig.  5 
were  cut  through  parallel  to  the  face  b,  one  of  the  parts 
revolved  180  degrees  around  a 
vertical  axis  parallel  to  the  edge 
mb  and  then  joined  and  the  two 
parts  pushed  together  so  that 
they  mutually  penetrate.  In 
Fig.  9  the  same  arrangement  is 
seen  looking  down  on  the  face  b 
of  the  crystal;  acya  is  the  outline 
of  the  original  crystal;  if  this  is 
cut  out  in  a  piece  of  paper  and  then  turned  over  180 
degrees  or  upside  down  and  laid  on  acya  so  that  the  edges 
aa  are  brought  together,  it  will  give  the  result  seen  in 
the  figure.  In  the  twin  crystal  illustrated  in  Fig.  8  the 
face  c  slopes  toward  the  observer,  the  face  y  slopes  away 
behind;  in  the  twinned  half  this  is  reversed;  as  explained 
under  the  cleavage  of  feldspars  this  fact  is  of  importance 
in  helping  to  recognize  these  twins  when  the  outward 
crystal  form  is  imperfect  or  wanting.  Carlsbad  twins 
of  the  character  described  are  found  of  all  the  different 
varieties  of  feldspar;  they  are  most  perfectly  developed 
in  the  phenocrysts  of  the  porphyritic  igneous  rocks, 
especially  in  the  large  orthoclase  phenocrysts  of  some 
granite  porphyries. 

In  the  Carlsbad  twin  the  plane  of  division  of  the  two 
parts  is  one  parallel  to  the  face  b;  the  axis 
on  which  one  part  is  revolved  is  the  vertical 
line  parallel   to  the  edge  ab  of  Fig.  10  and 
not  one  perpendicular  to  b  or  parallel  to  the 
edge  ac  which  is  usually  the  case  in  twinning, 
as  already  mentioned.     The  face  c  in  ortho- 
clase makes  a  right  angle  with  6;  the  outline 
of  the  face  a  is,  therefore,  a  rectangle,  and  if  the  crystal 
were  divided  along  the  dotted  line  by  a  plane  parallel  to  6 
and  one  of  the  halves  revolved  180  degrees  on  an 


Fig.  xo 


38 


ROCKS  AND   ROCK  MINERALS 


parallel  to  the  edge  ac,  that  is,  perpendicular  to  6,  it  would 
appear  precisely  as  before  and  no  twinning  would  occur. 
The  crystallographic  reason  for  this  is  that  &  is  a  symme- 
try plane,  since  the  crystal  is  monoclinic,  and  a  symmetry 
plane  cannot  be  a  twinning  plane. 

In  the  plagioclase  group,  in  albite,  anorthite  and  their 
admixtures,  the  face  c  makes  an  oblique  angle  with  the 
face  6;  the  face  a  is,  therefore,  a  rhomboid  and  not  a 
rectangle  as  shown  in  Fig.  11:  if  this  crystal  is  divided 
along  the  dotted  line  and  one  of  the  halves  revolved  180 
degrees  it  will  present  the  appearance  seen  in  Fig,  12; 
the  face  c  and  the  lower  c  now  brought  on  top  slope  toward 


M 


Fig.  ii 


Fig.  12 


Fig. i3 


each  other,  forming  a  re-entrant  angle,  while  below  they 
produce  a  salient  angle.  A  twin  crystal  is,  therefore, 
produced,  and  this  kind  of  twinning  is  known  as  the  albite 
method  because  it  is  so  generally  found  in  that  variety 
of  feldspar.  A  complete  crystal  of  this  kind  is  seen 
in  Fig.  13.  The  crystallographic  reason  that  this  can 
occur  is  because  these  feldspars  are  triclinic;  they  have, 
therefore,  no  symmetry  plane,  and  any  one  of  the  faces 
might  serve  as  a  twinning  plane. 

Multiple  Twinning.  In  nature,  in  actual  practice,  we 
rarely  find  a  single  albite  twin  of  the  kind  described  above. 
In  the  rock-making  plagioclases  the  crystals  are  divided 
again  and  again  into  thin  slices,  and  these  are  alternately 
twinned  upon  one  another,  producing  the  effect  seen  in 
Fig.  14.  Indeed,  this  albite  twinning  descends  to  such 
a  remarkable  degree  of  fineness  that  the  twin  layers  are 


ROCK-MAKING   MINERALS 


39 


less  than  the  one  hundred  thousandth  of  an  inch  in  thick- 
ness and  are  scarcely  to  be  perceived  in  thin  sections  in 
polarized  light  under  the  highest  powers  of  the  micro- 
scope. It  frequently  happens,  however,  especially  in 
those  feldspars  containing  much  lime,  like  labradorite, 
that  it  is  coarse  enough  to  be  readily  seen  by  the  naked 
eye;  one  cleavage  surface  of  such  a  feldspar  appears  as  if 


Fig. 


Fig.  15 


ruled  by  fine  parallel  lines  or  striations  as  illustrated  in  Fig. 
15.  Even  when  very  fine  and  on  a  small  cleavage  surface  of 
a  feldspar  grain  embedded  in  the  rock,  by  a  proper  adjust- 
ment of  the  light  reflected  from  the  surface  and  the  use  of 
a  good  lens  this  multiple  twinning  may  be  distinctly  seen. 
Sometimes  feldspars  are  twinned  both  according  to 
the  Carlsbad  and  the  albite  laws;  they 
may  be  seen  divided  into  the  Carlsbad 
halves  by.  the  reflection  of  light  from 
the  cleavage  and  each  of  these  ruled 
by  the  fine  lines  of  albite  twinning. 
An  illustration  of  the  combination  of 
these  two,  each  Carlsbad  half  divided 
into  albite  halves,  is  seen  in  Fig.  16. 
The  practical  use  of  the  twinning  of  feld- 
spars is  explained  in  the  paragraph  on  methods  for  their 
determination.  Other  methods  of  twinning  beside  those 


Fig.  16 


40         ROCKS  AND  ROCK  MINERALS 

mentioned  occur  in  the  feldspars,  but  in  the  megascopic 
study  of  rocks  they  are  not  of  importance. 

Cleavage.  All  the  different  varieties  of  feldspar  are 
alike  in  possessing  a  good  cleavage  in  two  directions,  one 
parallel  to  the  face  c  and  another  parallel  to  b  (see  Fig. 
7).  Since  in  orthoclase  these  two  faces  intersect  at  a 
right  angle,  so  also  do  the  cleavages,  and  from  this  fact  its 
name  is  derived  (Greek,  op0o<s}  straight,  right  -f-  K\dv, 
to  break) ;  in  the  lime-soda  feldspars,  albite  to  anorthite, 
these  faces  are  slightly  oblique,  and  so  are  the  cleavage 
planes;  hence  the  name  plagioclase  (Greek,  7r\ayios} 
oblique  +  K\av}  to  break)  has  been  given  to  the  group. 

In  rocks,  if  the  feldspar  grains  are  of  good  size,  the 
cleavages  are  readily  seen  by  reflected  light;  they  are  com- 
monly interrupted,  giving  rise  to  steplike  appearances. 
Even  when  the  grains  are  small  the  cleavage  can  usually  be 
detected  with  a  lens  in  good  light.  Sometimes  when  the 
feldspars  are  more  or  less  altered,  as  described  under 
alteration,  they  lose  more  or  less  completely  their  capacity 
for  showing  good  cleavage  faces  on  a  broken  surface  of 
the  rock,  and  this  fact  must  be  taken  into  account  in 
making  determinations.  As  in  the  crystals  which  show 
distinct  faces,  so  in  cleavage  pieces,  the  amount  of  obliquity 
of  the  plagioclases  is  too  small  to  be  used  in  distinguishing 
them  from  right  angled  orthoclases  by  the  eye  or  lens. 

On  a  fractured  rock  surface  if  the  crystal  grains  are  of 
sufficient  size  the  cleavages  frequently  permit  one  to 
recognize  that  they  are  twinned  according  to  the  Carlsbad 
method.  The  grain  or  broken  crystal  appears  divided 
into  two  parts  by  a  distinct  line;  on  one  side  of  this,  if  the 
line  points  away  from  the  observer,  the  cleavage  sur- 
faces slope  or  step  away  in  one  direction ;  in  the  other  half 
they  slope  towards  the  observer  at  an  equal  angle,  like 
the  two  c  faces  in  Fig.  8,  to  which  indeed  they  are 
parallel.  This  can  usually  be  readily  seen  by  shifting  the 
position  of  the  surface  in  a  good  light  until  the  cleavages 
reflect  it.  At  the  same  time  if  examined  with  a  good 


ROCK-MAKING   MINERALS  41 

lens  they  may  often  be  seen  to  be  ruled  by  the  fine  parallel 
striations  of  the  albite  twinning,  which  indicates  that  the 
feldspar  grain  is  a  plagioclase. 

Fracture.  In  directions  in  which  they  do  not  cleave 
the  fracture  of  feldspars  is  uneven  and  sometimes  some- 
what conchoidal.  They  are  brittle. 

Color,  Luster  and  Streak.  Feldspars  do  not  possess 
any  natural  color,  hence,  as  explained  under  the  color  of 
minerals,  they  should  normally  be  either  limpid  and  color- 
less or  white.  Transparent,  colorless,  glassy  feldspars  in 
rocks  are  confined  to  fresh  and  recent  lavas  in  which  they 
may  be  frequently  seen  in  the  phenocrysts;  they  practi- 
cally never  occur  in  massive  granular  rocks  like  granites, 
gneisses,  etc.  In  such  lavas  the  luster  may  be  strongly 
vitreous.  More  commonly  they  are  semi-translucent  or 
opaque  and  white,  grayish  white  or  yellowish  and  of  a 
somewhat  porcelain-like  appearance.  Orthoclase  and 
the  alkalic  group  of  feldspars  in  general  are  very  apt  to 
have  a  tinge  of  red;  this  color  varies  from  a  pale  flesh 
color  to  a  strong  brick-red  or  brownish  red;  a  distinct 
flesh  color  is  the  shade  most  common.  It  is  this  which 
gives  many  granites  used  for  building  stones  their  color. 
It  is  most  probable  that  this  variety  of  color  is  caused  by 
finely  disseminated  ferric  oxide  dust  which  acts  as  a  pig- 
ment, and  it  must  be  considered  as  exotic  and  not  a  natural 
color.  The  plagioclases  or  lime-soda  feldspars  more  rarely 
show  this;  they  are  commonly  gray,  and  the  difference 
between  the  two  classes  of  feldspars  is  apparently  due  to 
a  difference  in  the  chemical  behavior  of  iron  towards  soda 
and  potash;  soda  enters  readily  into  combination  with 
iron  in  silicate  minerals,  while  potash  does  not.  Thus 
in  the  potash  feldspars  the  iron  would  tend  to  be  present 
as  free  oxide  and  color  them.  Therefore  rocks  with 
potassic  feldspars  often  tend  to  be  of  reddish  color,  those 
with  sodic  feldspars  tend  to  be  gray.  This  distinction 
may  be  used  to  some  extent  as  an  indicator  of  the  kinds 
of  feldspar,  but  it  must  never  be  taken  as  an  absolute 


42  ROCKS  AND    ROCK  MINERALS 

rule,  because  many  potassic  feldspars  are  white  or  gray, 
and  conversely  many  instances  occur  where  rocks  with 
soda-lime  feldspars  are  red.  In  general  one  may  say 
that  if  the  rock  contains  two  feldspars  one  of  which  is  red 
while  the  other  is  not,  it  is  almost  certain  that  the  red 
feldspar  is  a  potassic  one  or  orthoclase. 

The  potassic  feldspars,  especially  the  variety  called  microcline 
when  occurring  in  distinct  crystals  in  the  miarolitic  druses  of  granitic 
rocks,  have  sometimes  a  green  color,  pale  to  bright  grass-green. 
This  is  also  an  exotic  coloration  and  is  supposed  to  be  due  to  some 
organic  substance  acting  as  a  pigment,  since  it  disappears  on  heating. 

Sometimes  the  rock  feldspars  are  gray,  dark,  smoky  or  bluish- 
gray  or  even  black.  While  this  may  happen  with  alkalic  varieties^ 
it  is  much  more  common  with  the  soda-lime  ones,  especially  lab- 
radorite.  It  is  caused  by  a  fine  black  dust  disseminated  through 
them  which  acts  as  a  pigment  and  which  may  sometimes  be  mag- 
netite dust,  but  is  much  more  often  ilmenite,  —  titanic  iron  ore. 
Pine  examples  of  these  are  seen  in  the  labradorite  rocks  from  Canada, 
the  Adirondack  region  in  New  York  State  and  from  Labrador  which 
have  been  called  anorthosites.  Sometimes  these  inclusions  are  of 
sufficient  size  and  so  regularly  arranged  in  the  feldspar  that,  by  the 
interference  of  light,  they  produce  an  opalescence  or  play  of  colors 
in  the  mineral  as  seen  in  the  beautiful  examples  from  St.  Paul's 
Island  on  the  coast  of  Labrador  and  from  Kiev  in  Russia. 

In  other  cases  feldspars  have  a  pearly  bluish  opalescence  from 
innumerable  minute  cracks  regularly  arranged  which  reflect  light 
with  interference  colors. 

The  luster  is  vitreous  and  on  cleavages  often  pearly. 
Feldspars  which  are  more  or  less  altered  often  have  a 
waxlike  appearance  and  a  waxy,  glimmering  luster;  if 
completely  altered  they  may  look  earthy  and  have  no 
luster. 

The  streak  is  white  and  not  characteristic. 

Hardness.  This  is  6.  Scratched  by  quartz,  scratches 
glass,  but  is  not  scratched  by  the  knife. 

Specific  Gravity.  Orthoclase  =  2.55,  albite  =  2.62, 
anorthite  •=  2.76.  That  of  the  various  mixtures  varies 


ROCK -MAKING  MINERALS 


43 


between  these  limits;  thus  the  alkalic  feldspars  which  con- 
sist of  a  mixture  of  orthoclase  and  albite  average  about 
2.57,  while  the  plagioclases  vary  regularly  with  the  relative 
amounts  of  soda  and  lime,  that  of  labradorite  being  2.67. 
If  the  specific  gravity  of  a  fragment  of  feldspar  can  be 
taken  with  accuracy  to  the  second  place  of  decimals  it 
affords  a  fairly  good  rough  method  of  ascertaining  its 
composition. 

Chemical  Composition.     This  is  shown  in  the  following 
table. 


SiO2 

A1203 

CaO 

Na2O 

K2O 

Total. 

I 

64.7 

18.4 

16.9 

100 

II 

68.7 

19.5 

11.8 

100 

III 

43.2 

36.7 

20.1 

100 

IV 

55.6 

28.3 

10.4 

5.7 

100 

V 

66.7 

18.9 

5.7 

8.7 

100 

I,  Orthoclase  (and  microcline);  II,  Albite;  III,  Anorthite;  IV, 
Labradorite  (equal  mixture  of  albite  and  anorthite);  V,  Alkalic 
feldspar  (equal  mixture  of  orthoclase  and  albite). 

The  mixtures  vary  naturally  with  the  proportions 
of  the  pure  products;  examples  of  equal  parts  are  given 
in  IV  and  V.  The  other  substances,  such  as  iron  oxide, 
etc.,  shown  in  feldspars  by  chemical  analyses,  are  due 
to  impurities. 

Blowpipe  and  Chemical  Characters.  A  fine  splinter 
fuses  before  the  blowpipe  with  difficulty  to  a  globular 
ending,  more  easily  with  anorthite  and  the  varieties  rich 
in  lime  than  with  albite  and  orthoclase.  The  flame  shows 
the  persistent  yellow  coloration  of  soda;  only  occasionally 
in  the  rock  feldspars  does  orthoclase  occur,  which  is  pure 
enough  to  give  the  violet  flame  of  potash.  Orthoclase 
and  albite  are  not  acted  upon  by  ordinary  acids  to  an 
appreciable  extent;  as  the  feldspars  increase  in  lime  they 


44  ROCKS  AND  ROCK  MINERALS 

become  more  soluble,  thus  labradorite  is  very  slowlj 
dissolved  while  anorthite  is  slowly  dissolved  and  affords 
gelatinous  silica. 

Alteration.  Under  the  action  of  various  agencies  the 
feldspars  are  prone  to  alter  into  other  substances,  which 
depend  in  part  on  the  nature  of  the  agents  and  in  part  on 
the  composition  of  the  feldspar  attacked.  Some  of  these 
changes  and  products  are  quite  complex  and  their  nature 
and  significance  have  not  as  yet  been  sufficiently  studied 
for  us  to  understand  them,  but  some  of  the  simpler  and 
more  important  ones  are  as  follows. 

When  the  feldspars  are  acted  upon  by  water  carrying 
carbonic  acid  gas  in  solution,  which  may  be  the  case  in 
surface  waters  leaching  downward  or  in  hot  waters  rising 
from  depths  below,  they  may  be  turned  into  kaolin  or 
muscovite  with  separation  of  free  silica  and  alkaline 
carbonates.  These  changes  may  be  expressed  chemically 
as  follows. 

Orthoclase  4-  Water  +  Carb.  diox.    =       Kaolin        +  Quartz   +Potas.  Carb. 

2KAlSi3O8  +2H2O  +         CO2         =  H4AI2Si2O9    +    4SiO2  +      K2CO3 

Orthoclase  +  Water  +   Carb.  diox.  =     Muscovite    +  Quartz.  +  Potas.  Carb. 

3KAlSi3O8  +   HaO     +         CO2        =H2K(AlSiO4)3+    6  SiO2   +      K2CO3 

What  determines  whether  the  removal  of  the  potash  from  the 
feldspar  will  be  complete  so  that  kaolin  is  formed  or  only  partial 
so  that  muscovite  is  the  resultant  product  is  not  clearly  understood. 
In  a  general  way  one  may  say  that  weathering  from  the  action  of 
surface  waters  generally  forms  kaolin  while  the  change  to  muscovite 
is  more  apt  to  be  a  deep-seated  affair  and  is  especially  noted  in 
processes  of  metamorphism.  In  mines  it  is  often  seen  that  the 
solutions  which  deposited  the  ores  have  altered  the  rocks  enclosing 
them,  sometimes  to  kaolin,  sometimes  to  a  form  of  muscovite  (sericite) 
and  sometimes  to  other  products.  It  is  due  to  this  in  great  part 
that  such  rocks  are  so  often  changed  from  their  original  fresh  con- 
dition. 

All  feldspars  undergo  similar  changes  to  those  men- 
tioned, but  in  those  which  contain  lime  they  are  more 
complex,  as  calcite,  the  carbonate  of  lime  is  also  formed. 
Accordingly,  as  this  change  to  muscovite  or  kaolin  is  more 


ROCK-MAKING  MINERALS  45 

or  less  complete,  the  feldspars  lose  their  original  bright 
appearance  and  become  dull  and  earthy  in  character;  if  it 
is  pronounced  they  are  soft  and  may  be  cut  or  scratched 
with  the  knife  or  even  with  the  finger  nail.  In  certain 
changes  in  the  lime-soda  feldspars  they  have  a  faint, 
glimmering  luster,  are  semi-translucent,  often  of  a  pale 
bluish  or  grayish  tone,  lose  to  a  great  extent  their  property 
of  cleavage  and  resemble  wax  or  paraffin  as  mentioned 
under  cleavage.  Often  these  changes  do  not  take  place 
regularly  through  the  whole  mass  of  the  crystal,  some- 
times the  border  is  altered,  sometimes  the  center  only  is 
attacked  and  sometimes,  especially  in  the  lime-soda  ones, 
like  labradorite,  zones  between  the  two  are  altered.  If 
the  feldspars  of  a  rock  do  not  show  bright,  glistening 
cleavage  surfaces  it  may  be  considered  practically  certain 
that  they  are  more  or  less  altered.  These  alterations  of 
the  feldspars  are  of  great  importance  in  geologic  processes 
and  especially  in  the  formation  of  soils. 

In  addition  to  these  alterations  others  are  also  known,  thus  under 
some  circumstances  the  feldspars  are  changed  into  zeolites  and  in 
metamorphic  processes  those  containing  lime  may  take  part  with 
other  minerals  in  forming  epidote,  garnet,  etc.,  changes  which  are 
mentioned  elsewhere. 

Occurrence.  The  feldspars  are  of  wide  distribution, 
perhaps  more  so  than  any  other  group  of  minerals.  They 
are  found  in  all  classes  of  rocks,  in  most  of  the  igneous 
ones,  such  as  granites,  syenites,  porphyries  and  felsite 
lavas;  in  the  sedimentary  ones  in  certain  kinds  of  sand- 
stones and  conglomerates  and  in  the  metamorphic  rocks 
in  gneisses.  Since,  so  far  as  our  knowledge  extends,the 
crust  of  the  earth,  underlying  all  the  sedimentary  beds  of 
all  ages  deposited  upon  it,  is  composed  chiefly  of  granites, 
gneisses,  etc.,  in  which  feldspars  are  the  main  minerals, 
it  is  not  too  much,  perhaps,  to  say  that  there  is  more 
feldspar  in  the  world  than  any  other  substance  of  whose 
occurrence  we  have  knowledge. 


46  ROCKS  AND  ROCK  MINERALS 

Determination.  In  general,  the  two  cleavages  at  right 
angles  or  nearly  so,  the  vitreous  luster,  light  color  and 
hardness,  which  resists  the  point  of  the  knife,  enable  one 
in  the  field  to  recognize  the  feldspar  grains  of  rocks  and  to 
distinguish  them  from  the  other  common  minerals, 
especially  quartz,  with  which  they  are  usually  associated. 
Sometimes  the  crystal  form  may  also  be  of  assistance, 
especially  in  porphyries.  In  addition  one  or  more  of  the 
various  chemical  and  physical  properties  enumerated 
above  may  be  determined  on  separated  fragments,  if  the 
feldspar  grains  or  masses  are  of  sufficient  size. 

The  determining  of  the  different  varieties  of  feldspar  which  may  be 
present  in  a  rock  is,  however,  a  much  more  difficult  task  when  only 
megascopic  means  are  employed.  Sometimes  the  remarks  made 
under  the  heading  of  color  will  be  of  assistance.  If  the  cleavage 
surfaces  are  closely  examined  with  a  lens  and  the  fine  lines  of  stria- 
tion  of  the  albite  twinning  are  found  then  one  knows  that  a  plagio- 
clase  feldspar  is  present,  since  orthoclase  cannot  have  this  twinning 
as  previously  explained.  The  only  practical  exception  to  this  rule 
is  that  the  large,  often  huge,  crystals  of  potash  feldspar  found  in 
granite-pegmatite  dikes  are  often  not  really  orthoclase  but  micro- 
cline,  a  tricJinic  variety  and  a  good  cleavage  surface  of  this  ex- 
amined in  a  strong  light  with  a  powerful  lens  frequently  shows  a 
minute,  scarcely  perceptible,  multiple  twinning  like  the  albite 
twinning. 

If  no  multiple  twinning  is  seen  it  would  not  be,  therefore,  safe 
to  conclude  that  the  feldspar  is  necessarily  an  orthoclase  or  alkalic 
variety  and  not  a  plagioclase  because  this  twinning,  as  already 
stated,  is  often  so  fine  that  it  cannot  be  detected  with  the  lens  and  is 
sometimes  wanting.  As  the  grain  of  rocks  grows  finer  it  becomes 
increasingly  difficult  to  detect,  but  a  good  training  of  the  eye  by 
studying  a  series  of  rocks  in  which  it  is  present  in  the  feldspars  is  a 
great  help  and  eventually  enables  one  to  perceive  it  clearly  in  cases 
where  at  first  it  could  not  be  seen.  The  modern  tendency  on  the 
part  of  geologists  to  refer  all  difficulties  in  rocks  to  microscopic 
examination  of  thin  sections  has  led  to  a  great  neglect  in  the  training 
of  the  eye  for  megascopic  determination  of  minerals  in  rocks  with  a 
corresponding  loss  of  efficiency  in  the  field. 

If  the  albite  twinning  is  clearly  seen  in  several  of  the  feldspar 
grains  of  a  rock  it  may  be  quite  safely  concluded  that  a  considerable 
proportion  of  plagioclase  is  present  and  this  may  indeed  be  prac- 


ROCK-MAKING  MINERALS  47 

tically  the  only  feldspar  present.     If  it  cannot  be  seen  plagioclase 
may  or  may  not  be  present. 

Other  means  which  may  be  resorted  to  are  the  determination  of 
the  specific  gravity,  the  behavior  before  the  blowpipe,  and  with 
acids,  as  previously  mentioned,  and  the  chemical  tests  for  soda, 
potash  and  lime,  which  suggest  themselves  to  those  experienced  in 
analytical  chemistry.  Further  information  in  the  subject  should  be 
sought  in  the  special  manuals  devoted  to  determinative  mineralogy. 

THE  FELDSPATHOID   GROUP. 

The  feldspathoid  group  owes  its  name  to  the  fact,  that, 
like  the  feldspars,  it  is  composed  of  minerals  which  are 
silicates  of  alumina  with  soda,  potash  and  lime  and  that 
they  are  found  in  the  same  associations,  accompanying 
or  replacing  feldspars  and  playing  a  similar  function  in 
the  making  of  rocks.  Unlike  feldspars  they  are  com- 
paratively rare  and  are  restricted  entirely  to  certain  kinds 
of  igneous  rocks  such  as  nephelite  syenite.  Thus  in 
treating  of  the  occurrence  of  common  rocks  they  are, 
compared  with  the  feldspars,  of  relatively  much  less 
importance,  but,  in  dealing  with  questions  regarding  the 
origin  of  igneous  rocks,  they  are  of  great  significance. 
The  more  important  members  of  the  group  are  nephelite 
and  sodalite,  less  common  ones  are  noselite  and  hauynite, 
cancrinite  and  leucite. 

Nephelite.  This  mineral  crystallizes  in  short,  thick, 
hexagonal  prisms  or  tables  with  a  flat  base  and  top  but  it 
rarely  shows  distinct  crystal  form  in  rocks.  Most  com- 
monly it  occurs  in  shapeless  masses  and  grains  like  quartz. 
Its  normal  color  is  white,  but  it  is  usually  gray,  varying 
from  light  smoky  to  dark  in  tone,  sometimes  it  is  flesh 
colored  or  brick-red.  The  white  color  may  shade  into 
yellowish,  the  gray  into  bluish  or  greenish.  Streak,  light 
—  not  characteristic.  Translucent.  Its  luster,  when 
fresh,  is  oily  or  greasy  and  much  like  that  of  quartz  and, 
like  this  mineral,  it  has  no  good  cleavage  and  its  fracture 
is  somewhat  conchoidal.  Brittle.  Hardness,  nearly  that 
of  feldspar  =  6.  Specific  gravity,  2.55-2.61.  Its  com- 


48         ROCKS  AND  ROCK  MINERALS 

position  is  chiefly  NaAlSiO4  with  a  small  varying  amount 
of  potash  replacing  soda.  Before  the  blowpipe  a  fine 
splinter  fuses  quite  readily  to  a  globule  tingeing  the  flame 
deep  yellow.  Readily  soluble  in  dilute  acid  with  forma- 
tion of  gelatinous  silica. 

Sodalite.  The  form  of  crystallization  is  the  isometric 
dodecahedron,  so  often  seen  in  garnet,  but  this  rarely 
occurs  in  rocks,  the  mineral  commonly  occurring  in  form- 
less grains  and  lumps.  It  is  sometimes  white,  pink,  or 
greenish  gray,  but  the  usual  color  is  a  blue  of  some  shade, 
often  a  bright  sky-blue  to  dark  rich  blue.  The  blue  color 
may  be  due  to  a  slight  admixture  of  the  lapis-lazuli 
molecule  acting  as  a  pigment.  Usually  translucent. 
Cleavage  dodecahedral  but  not  striking  as  a  megascopic 
property;  fracture  uneven  to  poorly  conchoidal.  Luster 
vitreous  to  greasy.  Streak,  white.  Hardness  nearly 
that  of  feldspar,  5.5-6.  Specific  gravity,  2.15-2.30.  Its 
composition  is  Na4(AlCl)Al2(Si04)3  and  this  may  also  be 
expressed  3  NaAlSi04  .  NaCl,  but  it  should  not  be  under- 
stood from  this  that  it  consists  of  a  mixture  of  nephelite 
and  common  salt  molecules;  it  is  a  definite  chemical  com- 
pound into  which  the  chlorine  enters.  Fuses  rather 
easily  before  the  blowpipe  with  bubbling,  coloring  the 
flame  yellow.  Easily  soluble  in  dilute  acids  with  forma- 
tion of  gelatinous  silica;  in  the  nitric  acid  solution  chlorine 
may  be  tested  for  with  silver  nitrate. 

The  other  feldspathoids  are  less  common  and  in  their 
general  properties,  modes  of  occurrence  and  functions  as 
rock  minerals  are  similar  to  nephelite  and  sodalite,  which 
they  are  usually  found  associated  with  or  in  part  replacing 
in  those  rocks  in  which  they  occur. 

Hauynlte  and  Noselite.  These  show  characters  like  sodalite  but 
they  differ  from  it  in  containing  the  radical  SO3  of  sulphuric  acid  in 
the  place  of  chlorine  and  the  best  method  of  detecting  them  is  by 
the  test  for  sulphuric  acid  with  barium  chloride  in  their  nitric  acid 
solution.  They  differ  from  one  another  only  that  in  hauynite  a 
part  of  the  soda  is  replaced  by  lime  while  noselite  is  the  pure  soda 
compound.  Cancrinite  is  much  like  nephelite  in  its  genera!  prop- 


ROCK-MAKING  MINERALS  49 

erties,  it  contains  CO2  in  combination,  which  affords  aid  in  detecting 
it  as  explained  later  in  testing  minerals  and  rocks;  its  formula  might 
be  written  8  NaAlSiO4  .  CaCO3  .  CO2 .  3  H2O,  but  as  in  sodalite  it 
is  not  a  mixture  of  molecules  but  a  definite  compound.  The  color 
is  variable  but  frequently  a  bright  yellow  to  orange  which  may  also 
help  in  detecting  it.  It  is  supposed  at  times  to  be  caused  by  the 
alteration  of  nephelite,  but  in  most  cases,  if  not  always,  it  is  an 
original  mineral  crystallizing  from  a  molten  magma,  like  nephelite 
and  feldspar. 

Leucite  is  a  rare  feldspathoid  crystallizing  in  isometric  trape- 
zohedrons,  a  form  illustrated  in  garnet;  the  crystals  when  imperfect 
appear  spherical.  Its  cleavage  is  imperfect;  fracture  conchoidal; 
color  white  to  gray;  luster  vitreous.  Hardness  is  5.5-6;  specific 
gravity,  2.5.  Before  the  blowpipe  it  is  infusible  and  when  mixed 
with  powdered  gypsum  gives  the  flame  the  violet  color  of  potassium. 
It  dissolves  in  acids  without  gelatinizing.  Its  composition  is 
KAl(SiO3)2.  It  occurs  almost  wholly  in  lavas  and  is  nowhere 
common  except  in  those  of  central  Italy,  where  the  magmas  are 
characterized  by  a  high  content  of  potash.  The  most  noted  occur- 
rence is  in  the  lavas  of  Vesuvius,  in  some  of  which  it  is  found  in  good- 
sized,  well-shaped  crystals  of  the  form  illustrated  in  Fig. Blunder 
garnet.  Large  crystals,  altered,  however,  to  other  minerals,  have 
been  found  in  certain  syenites  and  related  rocks  in  Arkansas,  Mon- 
tana, Brazil  and  elsewhere. 

Alteration.  The  feldspathoids,  like  the  feldspars,  are 
liable  to  alteration  from  the  processes  of  weathering  when 
exposed  to  the  atmosphere  and  to  the  action  of  fluids 
circulating  in  the  rocks  at  lower  levels.  They  become 
converted  into  kaolin  or  muscovite  and  also  very  com- 
monly into  zeolites.  The  latter  case  is  very  general;  all 
that  is  necessary  is  a  rearrangement  of  the  molecule  and 
the  assumption  of  water  and  silica;  hence  when  the  feld- 
spathoids are  heated  in  a  closed  glass  tube  they  are  very 
apt  to  yield  water.  Thus 

Nephelite  and  silica  and  water  yield          analcite. 
NaAlSiO4   +  SiO2    +  H2O     =     NaAl(SiO3)2 .  H2O. 

The  determination  of  the  feldspathoids  in  rocks  is  best  done  by 
chemical  means.  With  the  exception  of  leucite,  which  is  too  rare  a 
mineral  to  be  considered  except  in  very  unusual  cases,  they  yield 
gelatinous  silica  and  may  be  tested  for  as  described  later  under 


50  R,OCKS  AND  ROCK  MINERALS 

mineral  tests.  Nephelite  is  easily  confused  with  quartz  which  it 
often  closely  resembles  in  rocks;  its  association  with  other  minerals 
and  the  appearances  of  those  rocks  in  which  it  chiefly  occurs  and 
which  are  described  in  their  appropriate  places,  helps  in  arousing 
suspicion  of  its  presence  and  this  is  readily  confirmed  by  its  solubility 
in  acids.  Fortunately  for  field  determinations  nephelite  is  a  very 
rare  mineral,  quartz  an  exceedingly  common  one ;  thus  the  assump- 
tion that  the  mineral  is  quartz  in  the  vast  majority  of  cases  will  be 
right. 

MICAS. 

The  micas  form  a  natural  group  of  rock  minerals,  which 
is  characterized  by  great  perfection  of  cleavage  in  one 
direction,  and  by  the  thinness,  toughness  and  flexibility 
of  the  elastic  plates  or  laminae  into  which  this  cleavage 
permits  them  to  be  split.  For  practical  purposes  of 
megascopic  rock  study  and  classification  they  can  be 
divided  into  two  main  groups,  light  colored  micas  or 
muscovite  and  related  varieties,  and  dark  colored  biotite 
and  related  varieties. 

Form.  Micas  crystallize  in  six-sided  tablets  with  flat 
bases;  they  appear  to  be  short  hexagonal  prisms,  (see 
Fig.  17);  in  reality,  as  maybe  shown  by  optical  methods, 
their  crystallization  is  monoclinic.  Their  side  faces  are 
rough  and  striated,  the  flat  bases,  which  are  usually  cleav- 


Fig.  17  Fig.  x8 

age  faces,  bright  and  glittering.  Sometimes  two  of  the 
side  faces  are  much  elongated,  as  in  Fig.  18.  While 
distinct  crystal  form  is  often  observed  in  rocks,  par- 
ticularly the  igneous  ones,  the  micas  are  much  more 
commonly  seen  in  shapeless  flakes,  scales  or  shreds,  with 
flat,  shining,  cleavage  faces.  Sometimes  the  foliae  or 
leaves  are  curled  or  bent. 


ROCK-MAKING  MINERALS  51 

Cleavage.  This  has  been  already  mentioned.  It  is 
perfect  parallel  to  the  base  and  it  is  this  property  combined 
with  its  flexibility,  transparency  and  toughness  that 
makes  the  large  crystals  and  sheets  of  muscovite  found  in 
pegmatite  veins  so  useful  in  making  stove  windows,  lamp 
chimneys,  etc.,  where  ordinary  glass  is  easily  broken. 
Sometimes  when  the  mineral  occurs  in  an  aggregate  of 
minute  scales,  especially  muscovite  in  the  sericite  form, 
the  cleavage  is  not  so  apparent,  but  can  generally  be  seen 
by  close  observation. 

Color,  Luster  and  Hardness.  Muscovite  is  colorless, 
white  to  gray  or  light  brown,  often  with  greenish  tones. 
The  other  light-colored  micas  are  similar,  except  that 
lithia  mica  or  lepidolite,  found  in  pegmatite  veins,  is 
usually  pink  or  lilac  colored.  These  micas  in  thin  sheets 
are  transparent. 

Biotite  and  its  congeners  are  black,  in  thin  sheets 
translucent  with  strong  brown,  red-brown  or  deep  green 
colors.  The  phlogopite  variety  is  pale  brown,  sometimes 
coppery.  The  luster  of  micas  is  splendent,  on  cleavage 
faces  sometimes  pearly  and  in  the  sericite  variety  of 
muscovite  frequently  silky.  The  hardness  varies  from 
2-3;  all  are  easily  scratched  with  the  knife. 

Chemical  Composition.  Chemically  the  micas  which 
take  part  in  rock-making  may  be  divided  into  two  main 
groups,  one  containing  iron  and  magnesia,  of  which  the 
dark-colored  biotite  is  an  example,  the  other  devoid  of 
these  oxides,  of  which  muscovite  is  the  most  prominent 
member.  They  are  complex  in  composition,  silicates  of 
alumina  with  alkalies  and  containing  more  or  less  hydroxyl 
and  fluorine.  The  two  main  varieties  may  be  represented 
as  follows: 

Muscovite  =  H2KAl3(SiO4)3. 

Biotite        =  (HK)2(MgFe)2(AlFe)2(Si04)3. 

The  other  members  of  the  muscovite  group  are,  paragonite> 
a  rare   mineral   like   muscovite,  in  which  soda  replaces 


52 


ROCKS  AND  ROCK  MINERALS 


potash  and  lepidolite,  in  which  the  potash  of  muscovite  is 
partly  replaced  by  lithia.  In  the  biotite  sub-group, 
phlogopite  is  a  variety  nearly  free  from  iron  and  thus  a 
magnesia  mica;  the  lack  of  iron  accounts  for  its  lighter 
color;  lepidomelane,  on  the  contrary,  is  very  rich  in  iron, 
especially  ferric  oxide,  while  another,  zinnwaldite,  contains 
some  lithia  in  place  of  part  of  the  potash.  The  formulas 
of  these  compounds  are  very  complex  and  in  part  not 
absolutely  settled.  The  adjoining  table  of  analyses 
shows  the  chemical  differences  between  the  varieties. 


I 

II 

III 

IV 

V 

VI 

VII 

SiO2     

44.6 

46.8 

48.8 

36.0 

39.6 

32.1 

45.9 

A12  O3     

35.7 

40.1 

28.3 

18.8 

17.0 

18.5 

22.5 

Fe,  O, 

1.0 

0.3 

5.6 

0  3 

19.5 

0  6 

FeO     

1.0 

0.1 

14.7 

0.2 

14.1 

11  6 

MgO    

0.6 

9.8 

26.5 

1.0 

CaO     

0.1 

1.3 

0.1 

0.6 

Na2  O  

2.4 

6.4 

0.7 

0.4 

0.6 

1.5 

K2O    

9.8 

12.2 

9.3 

10.0 

8.1 

10.5 

Li2O   



4.5 

3.3 

H2O  

5.5 

4.8 

1.7 

2.5 

3  0 

4.6 

0  9 

F      

0.7 

5.0 

0.3 

2  2 

7.9 

X*  

1.9 

1  2 

1.4 

1.7 

Total  

100.8 

100.0 

101.7 

99.9 

100.6 

100.8 

104.9 

*  X  represents  small  quantities  of  non-essential  oxides  present. 

I,  Muscovite,  Auburn,  Me. ;  II,  Paragonite,  the  Alps ;  III,  Lepid- 
olite, Hebron.  Me.;  IV,  Biotite,  from  granite,  Yosemite,  Cal.;  V, 
Phlogopite,  Burgess,  Ontario;  VI,  Lepidomelane,  from  nephelite 
syenite,  Litchfield,  Me, ;  VII,  Zinnwaldite,  Zinnwald,  Erzgebirge. 

Blowpipe  and  Chemical  Characters.  Usually  the  micas 
whiten  before  the  blowpipe  and  fuse  on  the  edges,  when 
in  thin  scales.  Lepidomelane  fuses  to  a  black  magnetic 
globule.  Heated  in  the  closed  glass  tube  they  yield 
very  little  water,  which  helps  to  distinguish  them  from 


ROCK-MAKING  MINERALS  53 

chlorites  and  other  micaceous  rock  minerals.  When  thin 
scales  are  treated  with  a  little  boiling  concentrated  sul- 
phuric acid  in  a  test  tube,  muscovite  and  the  re- 
lated light-colored  kinds  are  scarcely  acted  upon,  but 
biotite  and  its  congeners  are  decomposed,  the  scales  losing 
their  luster  and  transparency  while  the  acid  becomes 
turbid.* 

Lepidomelane  is  soluble  in  hydrochloric  acid,  depositing 
silica  in  scales,  an  important  character  serving  to  dis- 
tinguish it  from  the  other  micas.  The  lithia  micas  impart 
a  red  color  to  the  blowpipe  flame,  paragonite  the  yellow 
color  of  sodium. 

Alteration.  Biotite  under  the  action  of  weathering 
changes  to  chlorite,  loses  its  elasticity  and  becomes  soft 
and  of  a  green  color.  Muscovite  being  itself  often  the 
product  of  various  alterations  of  other  minerals,  especially 
of  feldspars,  appears  well  fitted  to  withstand  the  process 
of  weathering  and  its  scales  often  occur  in  soils  made  of 
broken-down  rocks  whose  other  constituents  may  be 
greatly  changed.  It  eventually  changes,  loses  its  trans- 
parency and  elasticity  and  perhaps  becomes  ultimately 
converted  into  clay. 

Occurrence.  The  common  micas  are  minerals  of  wide 
distribution  as  rock  components.  Biotite  is  a  very 
common  and  prominent  ingredient  of  many  igneous  rocks, 
especially  of  those  rich  in  feldspar  like  granites  and 
syenites  —  in  ferro-magnesian  rocks  like  gabbro  it  is  less 
prominent;  it  is  also  seen  in  many  felsite  lavas  and  por- 
phyries. It  occurs  commonly  in  some  metamorphic 
rocks  such  as  gneisses  and  schists  and  is  frequently  one  of 
the  products  of  contact  metamorphism  of  igneous  rocks. 
From  its  liability  to  alteration  it  does  not  figure  as  a 
component  of  sedimentary  beds.  The  phlogopite  variety 
containing  little  iron  has  been  found  in  some  rare  cases  in 

*  Care  should  be  used  in  making  this  test  not  to  bring  the  hot 
acid  in  contact  with  water,  or  the  mixture  will  take  place  with 
explosive  activity. 


54         ROCKS  AND  ROCK  MINERALS 

igneous  rocks,  but  it  chiefly  occurs  as  a  product  of  meta- 
morphism  in  crystalline  limestones  or  impure  marbles 
and  dolomites.  Lepidomelane  and  zinnwaldite  appear  to 
occur  chiefly  in  granites  and  syenites,  especially  in  peg- 
matitic  varieties.  Muscovite  occurs  in  granites  and 
syenites,  especially  in  pegmatite  veins  and  in  miarolitic 
druses  and  in  places  where  the  igneous  rocks  have  been 
subjected  to  later  fumarole  actions  furnishing  water  and 
fluorine.  It  is  sometimes  seen  in  intrusive  porphyries 
and  lavas  of  felsitic  character.  It  is  especially  common 
in  the  metamorphic  rocks  and  is  widely  distributed  in 
gneisses  and  schists;  sometimes,  especially  in  the  latter 
rocks,  it  is  in  the  form  of  an  aggregate  of  minute  scales 
which  have  a  silky  luster  and  largely  lack  in  appearance 
the  evident  characters  of  the  mineral,  such  as  its  cleavage; 
this  variety  has  been  called  sericite.  When  feldspars  are 
altered  to  muscovite,  rather  than  to  kaolin,  this  sericite 
variety  is  the  common  product.  In  the  sedimentary 
rocks,  such  as  conglomerates  and  sandstones,  muscovite 
is  sometimes  seen,  an  unchanged  remnant  of  the  original 
rocks  from  which  their  material  came.  Lepidolite  is 
practically  restricted  to  granite-pegmatite  veins  and  is 
constantly  accompanied  by  tourmaline.  Paragonite  has 
been  found  in  only  a  few  cases,  in  schists,  playing  the  role 
muscovite  would  ordinarily  have. 

Determination.  From  the  ordinary  rock  minerals  the 
micas  are  at  once  distinguished  by  their  appearance,  high 
luster  and  eminent  cleavage,  the  latter  quality  and  their 
hardness  being  readily  tested  in  the  field  by  the  knife 
point.  From  the  chlorite  group  and  from  talc,  which 
resemble  them,  they  are  told  by  the  elasticity  of  their 
split-off  laminae,  those  of  the  chlorites  and  talc  being 
flexible  but  not  elastic.  From  chloritoid  a  micaceous 
appearing  mineral  of  a  gray  or  green  color,  a  hydrated 
silicate  of  alumina,  magnesia  and  iron,  which  is  sometimes 
seen  in  distinct  crystals  in  certain  metamorphic  rocks,  they 
are  readily  distinguished  by  its  superior  hardness  =  6.5 


ROCK-MAKING  MINERALS  55 

and  brittieness.  The  different  varieties  of  mica  are 
best  discriminated  by  the  chemical  and  blowpipe  tests 
already  mentioned. 

PYROXENES. 

The  pyroxene  group  embraces  a  number  of  important 
minerals  which  have  in  common  the  fact  that  they  are 
metasilicates,  salts  of  metasilicic  acid,  H2Si03,  in  which 
the  hydrogen  is  replaced  by  various  metals  as  shown 
later,  and  although  they  may  differ  in  the  system  in  which 
they  crystallize,  in  having  closely  related  crystal  form, 
notably  a  prismatic  cleavage  of  87  and  93  degrees.  As 
rock  minerals  they  are  of  greatest  importance  in  the 
igneous  rocks  though  they  may  be  prominent  at  times  in 
some  of  the  metamorphic  ones.  Some  igneous  rocks  are 
composed  almost  entirely  of  pyroxene. 

It  is  often  difficult  to  recognize  pyroxene  in  the  rocks 
and  distinguish  it  from  several  other  minerals  purely  by 
simple  megascopic  methods  and  largely  impossible  to 
tell  from  one  another  by  such  means  the  many  varieties 
recognized  by  mineralogists  and  petrographers.  The 
differences  between  these  varieties  are  chiefly  in  chemical 
composition  and  optical  properties  and  these  must  be 
determined  by  chemical  and  optical  methods. 

For  practical  megascopic  petrography  the  pyroxenes 
may  be  divided  into  the  following  sub-groups  dependent 
on  their  color,  behavior  before  the  blowpipe  and  chemical 
reaction  for  lime  as  described  later:  hypersthene,  diopside, 
common  pyroxene,  augite  and  aegirite. 

Form.  Hypersthene  crystallizes  in  the  orthorhombic, 
the  others  in  the  monoclinic  systems,  but  this  distinction 
is  not  a  matter  of  practical  importance  in  megascopic 
work,  since  the  former  is  rarely  well  enough  crystallized 
to  determine  the  system.  The  common  form,  in  which 
the  monoclinic  rock  pyroxenes  crystallize,  is  a  prism, 
usually  short  and  thick  though  sometimes  longer  and 
more  slender.  Such  a  prism  is  shown  in  Fig.  19,  the 


56 


ROCKS  AND  ROCK  MINERALS 


ends  modified  by  pyramidal  faces.  Generally,  however, 
the  edges  of  the  prism  mm  are  truncated  by  a  front  face 
a  and  a  side  face  b  —  sometimes  these  truncations  are 


Fig.  19 


Fig. 20 


Fig.  ax 


small  so  that  a  and  b  are  slender  (Fig.  20);  often  they 
are  very  broad  and  mm  narrow.  While  these  faces  are 
commonly  well  developed  and  often  lustrous  the  pyra- 
midal faces  are  often  very  imperfect  or  wanting,  the 
crystal  being  rounded  at  the  ends;  rarely  other  pyramidal 
faces  are  present  and  the  ends  much  more  complex  than 
in  the  figures.  The  augites  which  occur  in  igneous  rocks, 
especially  porphyries  and  lavas,  very  often  have  the 
appearance  and  development  shown  in  Fig.  21.  The  most 
important  thing  in  the  crystallization  is  that  the  angle  m 
on  TO  is  nearly  a  right  angle,  87  degrees,  so  that  the  prism 
is  nearly  square  in  cross  section  or  when  truncated  by  a 


Fig.  22 


and  b,  octagonal  as  shown  in  Fig.  22.  Besides  occurring 
in  prismatic  crystals  the  pyroxenes  also  are  very  common 
in  grains,  or  in  more  or  less  shapeless  masses;  this  is 


ROCK-MAKING  MINERALS  57 

usually  the  case  in  certain  massive  igneous  rocks  such  as 
gabbros  and  peridotites. 

Cleavage  and  Fracture.     As  previously  mentioned  the 
pyroxenes  have  a  cleavage  parallel  to  the  faces  mm,  nearly 
at  right  angles  as  shown  in  Fig.  23;  this  is 
fundamental  and  serves  to  distinguish  the 
mineral  from  hornblendes.    This  cleavage  is 
usually  very  good  but  not  perfect.      Some 
varieties  often  have  a  good  parting  in  other 
directions  resembling  cleavage  which  causes 
the  mineral  to  appear  lamellar,  perhaps  even 
somewhat  micaceous,  as  seen  in  the  pyroxenes  of  some 
gabbros.     Fracture  uneven;   the  mineral  is  brittle. 

Color  and  Luster.  The  color  varies  from  white  through 
various  shades  of  green  to  black,  according  to  the  amount 
of  iron  present.  The  pure  diopsides  are  white,  rarely 
colorless  and  transparent,  often  pale  green,  and  more  or 
less  translucent;  common  pyroxenes  are  dull  green  of 
various  shades;  augite  and  aegirite  are  black;  these  are 
opaque.  The  luster,  which  is  often  wanting,  is  glassy  to 
resinous.  Streak  varies  from  white  to  gray-green. 

Hardness  and  Specific  Gravity.  The  hardness  varies 
from  5-6.  Some  varieties  can  be  just  scratched  by  the 
knife.  The  specific  gravity  varies,  chiefly  with  the  iron 
present,  from  3.2-3.6. 

Chemical  Composition.  Pyroxenes  are  composed  of 
the  metasilicate  molecules  MgSiOs,  FeSiOs,  CaMg(SiC>3)2, 
CaFe(Si03)2,  NaFe(SiO3)2  and  RR2Si06  in  which  last 
R  =  MgFe  and  R  =  Al  and  Fe.  These  molecules  are 
isomorphous,  that  is,  capable  of  crystallizing  in  various 
mixtures  which  produce  the  same  crystal  form  and  many 
similar  physical  properties.  The  hypersthene  sub-group 
contains  mixtures  of  MgSiOg  and  FeSiOs  without  lime; 
diopside  is  CaMg(SiOs)2  with  little  or  none  of  the  iron 
molecule,  common  pyroxene  contains  variable  mixtures  of 
CaMg(SiO3)2  (diopside)  and  CaFe(SiO3)2  (hedenbergite) 
with  small  portions  of  the  other  molecules ;  augite  contains 


58 


ROCKS  AND  ROCK  MINERALS 


the  same  but  in  addition  a  large  amount  of  HR2SiOe; 
aegirite  is  mostly  NaFe(SiO3)2  and  is  thus  a  soda 
pyroxene. 

Blowpipe  and  Chemical  Characters.  Hypersthene  varies 
from  almost  infusible  in  the  blowpipe  flame  when  contain- 
ing little  iron  (variety  enstatite)  to  difficultly  so  with  much 
iron;  in  the  latter  case  it  becomes  black  and  slightly 
magnetic.  The  other  pyroxenes  are  much  more  fusible 
=  4  and  melt  quietly  or  with  little  intumescence  to  glassy 
globules  whose  color  depends  on  the  amount  of  iron,  diop- 
side  nearly  colorless,  common  pyroxene  green  or  brown, 
augite  and  aegirite  black;  the  last  two  magnetic.  Aegirite 
fuses  quietly  and  colors  the  flame  yellow.  They  are  but 
slightly  acted  upon  by  acids,  those  with  iron  more  so 
than  those  without. 

These  differences  in  the  chemical  composition  are  shown 
in  the  table  of  analyses. 


SiOa 

A1203 

Fe203 

FeO 

MgO 

CaO 

Na20 

X* 

Total. 

I  . 

53.1 

1.0 

17.8 

24.8 

2.7 

0.4 

99.8 

II. 

58.0 

1.3 

3.1 

36.9 

0.8 

100.1 

Ill 

55.1 

0.4 

1.1 

18.1 

25.0 

0.4 

0.2 

100.3 

IV. 

51.1 

2.0 

1.3 

12.3 

10.0 

22.1 

0.4 

99.2 

V  . 

47.0 

9.8 

4.5 

4.1 

16.0 

19.0 

100.4 

VI. 

51.4 

1.8 

23.3 

9.4 

0.3 

2.0 

11.9 

0.1 

100.2 

*  X  =  small  quantities  of  other  oxides. 

I,  Hypersthene,  Romsaas,  Norway;  II,  Hypersthene  (var.  ensta- 
tite), Bamle,  Norway;  III,  Diopside,  DeKalb,  N.  Y. ;  IV,  Common 
pyroxene,  Edenville,  N.  Y. ;  V,  Black  augite,  Vesuvius  lava;  VI, 
Aegirite,  from  syenite,  Hot  Springs,  Ark. 

Alteration.  The  pyroxenes  are  prone  to  alter  into 
other  substances  whose  nature  depends  partly  on  the 
kind  of  process  to  which  they  are  subjected  and  partly 
on  their  own  composition.  Thus  under  the  action  of 
weathering  they  may  be  converted,  if  containing  much 
magnesia,  into  serpentine,  or  into  chlorite,  if  containing 


ROCK-MAKING  MINERALS  59 

iron,  or  Into  both  and  often  carbonates  are  also  formed, 
such  as  calcite.  Those  containing  much  iron  may  com- 
pletely break  down  into  hydrated  iron  oxides,  such  as 
limonite,  and  carbonates. 

Another  very  important  change  is  one  which  they 
suffer  under  metamorphic  processes,  especially  regional 
ones.  In  this  they  become  altered  to  masses  of  fibrous, 
felty  or  stringy  hornblende  needles  and  prisms,  usually 
of  distinct  but  variable  green  colors.  This  process  is  of 
great  geologic  importance  for  by  means  of  it  whole  masses 
of  pyroxenic  rocks,  generally  of  igneous  origin,  such  as 
gabbros,  peridotites,  basalts,  etc.,  have  been  changed 
into  hornblendic  ones  to  which  a. variety  of  names,  such 
as  greenstone,  greenstone  schist,  hornblende  schist,  etc., 
have  been  applied.  The  process  is  further  mentioned 
under  metamorphism,  and  under  gabbro,  dolerite,  green- 
stone and  amphibolite. 

Occurrence.  The  pyroxenes  are  chiefly  found  in 
igneous  rocks,  especially  those  which  are  formed  from 
magmas  rich  in  lime,  iron  and  magnesia.  Therefore,  in 
the  dark  colored  rocks  of  this  class  they  should  always  be 
looked  for.  They  are  not  often  found  in  igneous  rocks 
which  contain  much  quartz,  hence  in  granites,  felsite 
porphyries  and  felsite  lavas  they  are  rare.  Augite  is 
found  in  basaltic  lavas  and  dark,  trap-like  intrusives, 
often  in  well  formed  crystals;  when  it  occurs  in  gabbros 
and  peridotites  it  is  commonly  in  grains  and  lumps. 
Hypersthene  is  prominent  in  masses  and  grains  in  some 
varieties  of  gabbro  and  peridotite.  Aegirite  occurs  chiefly 
in  nephelite  syenites  and  the  phonolite  variety  of  felsite 
lava.  Some  normal  syenites  and  related  rocks  contain 
diopside-like  or  common  pyroxene.  In  the  metamorphic 
rocks  common  pyroxene  and  diopside,  the  latter  some- 
times white  or  pale  greenish  and  transparent,  are  found 
in  impure  recrystallized  limestones  and  dolomites,  some- 
times in  well  formed  scattered  crystals,  sometimes 
aggregated  into  large  masses.  Common  pyroxene  also 


60         ROCKS  AND  ROCK  MINERALS 

occurs  in  some  gneisses.     Being  readily  decomposed  by 
weathering  they  play  no  part  in  sedimentary  beds. 

Determination.  If  the  mineral  under  examination  is 
in  well  formed  crystals  careful  observation  will  usually 
show  if  it  is  a  pyroxene  by  its  possession  of  the  forms 
previously  described.  The  outline  of  the  section  presented 
by  the  prisms,  especially  when  broken  across,  should  be 
noted  in  this  connection.  The  common  minerals  in  rocks 
with  which  pyroxenes  may  be  confused  are  hornblende, 
epidote  and  tourmaline.  The  lack  of  good  cleavage,  the 
superior  hardness,  the  high  luster,  dense  black  color  and 
triangular  shape  of  the  prism  cross  section  of  tourmaline 
readily  distinguish  it  from  pyroxene.  Epidote  has  one 
perfect  cleavage,  one  poor  one;  it  is  much  harder,  6-7; 
while  green  it  commonly  has  a  yellow  tone,  giving  a  yel- 
lowish green;  before  the  blowpipe  it  intumesces  when 
fusing.  The  distinction  of  pyroxene  from  hornblende  is 
more  difficult  and  is  treated  under  the  head  of  that 
mineral. 

To  distinguish  the  different  varieties  of  pyroxene  from  one  another 
the  blowpipe  tests  previously  mentioned  should  be  used  in  conjunc- 
tion with  the  natural  color  of  the  mineral.  The  hypersthenes  are 
most  certainly  told  from  other  pyroxenes  by  making  a  chemical  test 
to  prove  the  absence  of  lime  or  at  least  its  presence  in  only  minute 
quantity.  This  is  best  done  by  making  a  small  fusion  with  soda  as 
described  in  the  chapter  treating  of  mineral  tests. 

AMPHIBOLES  (HORNBLENDES). 

The  amphiboles,  or  hornblendes,  names  which  are  used 
interchangeably,  are  a  natural  group  of  silicate  minerals 
which  like  the  pyroxenes  are  salts  of  metasilicic  acid 
H2SiO3,  in  which  the  hydrogen  is  replaced  by  various 
metals  or  radicals.  They  have  in  common  a  certain 
crystal  form,  a  prismatic  cleavage  of  about  55  degrees, 
and  are  nearly  related  in  many  physical  properties.  As 
in  the  pyroxene  group,  to  which  the  amphiboles  are  closely 
allied  in  several  ways,  there  are  many  varieties  recognized 


ROCK-MAKING  MINERALS 


61 


by  petrographers,  dependent  upon  differences  in  chemi- 
cal composition  and  physical  properties,  especially  optical 
ones,  which  are  impossible  to  distinguish  by  the  eye  and 
many  of  them  indeed  by  ordinary  megascopic  tests. 

For  practical  work  in  megascopic  petrography  the 
amphiboles  may  be  divided  into  the  following  sub-groups: 
Tremolite,  Actinolite,  Common  Hornblende,  and  Arfved- 
sonite.  These  may  be  distinguished  by  their  colors,  asso- 
ciations and  behavior  before  the  blowpipe. 

Form.  Amphiboles  crystallize  in  the  monoclinic  sys- 
tem. The  crystals  are  usually  long  and  bladed,  formed 
by  two  prisms  mm  which  meet  at  angles  of  55  and  125 
degrees.  Sometimes  there  are  terminal  faces  rr  as  in  Fig. 
24,  sometimes  the  crystals  are  imperfect  at  the  ends  and 
no  terminal  faces  are  seen;  this  latter  is  common  in  rocks. 
Very  often  the  side  face  6  is  present  truncating  the  prism 


m 


Fig.  24 


Fig.  a5 


Fig.  a6 


Fig.  27 


edge  and  the  crystal  has  a  nearly  hexagonal  cross  section 
as  in  Fig.  25.  More  rarely  the  front  face  a  is  present  as  in 
Fig.  26.  The  black  hornblendes  found  as  phenocrysts  in 
some  basaltic  rocks  have  often  a  not  very  short  prism 
and  appear  as  in  Fig.  27;  these  are  the  hornblendes  which 
most  often  have  distinct  terminal  planes.  The  prismatic 
faces  mm  and  the  face  b,  if  it  is  present,  are  apt  to  be 
shining,  the  ends  are  frequently  dull.  It  is  not  common 


62         ROCKS  AND  ROCK  MINERALS 

for  amphibole  to  present  itself  in  rocks  in  crystals  whose 
planes  can  be  distinctly  seen;  when  this  occurs  it  is  mostly 
with  the  black  hornblendes  found  in  lavas  as  phenocrysts 
and  in  those  which  occur  in  limestones  and  dolomites 
which  have  been  subjected  to  metamorphism.  The 
common  appearance  is  in  long  slender  blades  with  irregu- 
lar, rough  ends;  this  is  usual  in  the  hornblende  schists 
where  the  crystals  are  aggregated  together  in  more  or  less 
parallel  position;  they  may  dwindle  in  size  to  shining 
needles,  becoming  so  fine  that  the  minute  prisms  can 
hardly  be  seen  with  the  lens;  the  aggregate  then  has  a 
silky  appearance.  In  the  felsitic  lavas  and  porphyries 
the  prisms  of  the  hornblende  phenocrysts  vary  from 
rather  short,  like  those  in  the  figures,  to  slender  needles; 
in  the  massive  doleritic  rocks  like  diorite  the  amphibole  is 
apt  to  occur  in  irregular  grains  and  small  masses.  Some- 
times as  in  asbestus  the  mineral  has  a  highly  developed 
columnar,  fibrous  form. 

Cleavage.     Amphiboles  have  a  highly  perfect  cleavage 
parallel  to  the  prism  faces  mm  as  illustrated  in  the  cross 
section,  Fig.  28.      Like  the  faces  mm 
these  cleavages  meet  at  angles  of  125 
and  55  degrees,  a  fact  of  great  import- 
ance  in  distinguishing  the  mineral .    The 
glittering  prismatic  faces  seen  on  the 
blades  and  needles  of  fractured  rock 
Pig.  as  surfaces    are    commonly  due  to   this 

good  cleavage.  The  fracture  is  uneven. 
Color  and  Luster.  The  color  varies  with  the  amount  of 
iron  from  white  or  gray  in  tremolite  to  gray-green  or 
bright  green  in  actinolite  to  darker  greens  and  black  in 
common  hornblende.  Arfvedsonite  is  black.  Some  var- 
ieties found  in  igneous  rocks  which  appear  black  are 
really  deep  brown.  The  mineral  varies  from  opaque  in  the 
deeper  colored  varieties  to  translucent  in  the  lighter 
ones.  The  luster  is  bright  and  vitreous  to  somewhat 
pearly  on  the  cleavage  surfaces;  in  very  fine  needle-like 


ROCK-MAKING  MINERALS  63 

or  fibrous  varieties,  silky.     Streak,  white  to  gray-green  or 
brownish. 

Hardness  and  Specific  Gravity.  The  hardness  varies 
from  5-6;  some  specimens  can  be  scratched  with  the 
knife.  The  specific  gravity  varies,  chiefly  with  the 
amount  of  iron,  from  2.9-3.5. 

Chemical  Composition.  Amphiboles  like  the  pyroxenes 
are  metasilicates,  salts  of  H2SiO3,  in  which  the  hydrogen 
atoms  are  replaced  by  calcium,  magnesium,  iron,  soda 
and  also,  as  shown  by  Penfield,  by  radicals  in  which 
alumina  plays  a  prominent  part  and  which  contain 
hydroxyl  (—  OH)  and  fluorine.  Penfield  has  also  shown 
that  when  calcium  is  present  it  replaces  one  fourth  of  the 
hydrogen  atoms.  Thus,  while  the  amphiboles  resemble 
the  pyroxenes  in  being  metasilicates  and  composed  of  the 
same  elements,  they  differ  from  them  in  being  much  more 
complex  and  in  containing  hydroxyl  and  fluorine.  Their 
compositions,  as  a  rule,  are  too  complicated  to  be  repre- 
sented by  simple  formulas,  but  in  a  general  way,  disregard- 
ing the  hydroxyl  and  fluorine,  one  may  say  that  each 
type  of  pyroxene  has  a  corresponding  amphibole  and  in 
this  connection  the  composition  of  the  pyroxenes  should 
be  studied. 

Thus  tremolite,  if  simply  represented  by  CaMgs  (8103)4 
corresponds  to  diopside  CaMg(SiOs)2:  while  actinolite, 

Ca(MgFe)3(Si03)4, 

with  variable  amounts  of  ferrous  iron  replacing  magnesium 
corresponds  to  common  pyroxene,  Ca(MgFe)  (8103)2; 
common  hornblende  or  hornblende  for  short  which  consists 
of  the  actinolite  molecule  with  others  in  which  radicals 
containing  alumina  or  ferric  iron  and  usually  both,  are 
present  and  perhaps  some  alkalies,  corresponds  in  general 
to  augite  which  is  a  variable  mixture  of  pyroxene  molecules 
with  alumina  and  ferric  iron;  arfvedsonite,  which  con- 
tains chiefly  soda,  lime  and  ferrous  iron,  plays  the  part  of 
aegirite,  the  soda  iron  pyroxene,  though  a  very  rare 


64 


ROCKS  AND  ROCK  MINERALS 


variety,  riebeckite,  more  nearly  corresponds  in  com- 
position. 

Glaucophane  is  a  rare  variety,  consisting  of  a  mixture  of  a  soda- 
alumina  molecule  with  a  hypersthene  molecule, 

NaAl(SiO3)2.  (FeMg)SiO3. 

It  is  distinguished  from  other  hornblendes  by  its  blue  color,  often  a 
rich  sky-blue  or  lavender-blue.  It  occurs  only  in  a  rare  variety  of 
hornblende-schists,  called  glaucophane-schists,  which  are  described 
under  amphibolites. 

The  chemical  composition  is  illustrated  in  the  following 
table  of  analyses. 


Si02 

A1203 

Fe203 

FeO 

MgO 

CaO 

Na2O 

H2O 

F2 

XyO* 

Total 

I 

57.5 

1.3 

0.2 

0.2 

24.9 

12.8 

0.7 

1.3 

0.8 

0.6 

100.3 

II 

56.1 

1.2 

0.8 

5.5 

21.2 

12.1 

0.2 

1.9 

0.1 

0.6 

99.7 

III 

41.9 

11.7 

2.5 

14.3 

11.2 

11.5 

2.7 

0.7 

0.8 

2.6 

99.9 

IV 

43.8 

4.4 

3.8 

33.4 

0.8 

4.6 

8.1 

0.1 

1.5 

100.5 

V 

55.6 

15.1 

3.1 

6.8 

7.8 

2.4 

9.3 

0.5 

100.6 

*  XyO  =  small  quantities  of  minor  components. 

I,  Tremolite,  Richville,  Gouverneur,  New  York;  II,  Actinolite 
Greiner,  Tyrol;  III,  Hornblende,  Edenville,  Orange  County,  New 
York:  IV,  Arfvedsonite,  Kangerdluarsuk,  Greenland;  V,  Glauco- 
phane, Island  of  Syra,  Greece. 

Blowpipe  and  Chemical  Characters.  Tremolite,  actino- 
lite  and  common  hornblende  melt  quietly  or  with  little 
intumescence  before  the  blowpipe,  fusing  rather  easily  at 
4.  The  color  of  the  bead  depends  on  the  amount  of  iron, 
tremolite  nearly  colorless,  actinolite  green  or  brown; 
common  hornblende  dark  and  shining.  Common  horn- 
blende sometimes  colors  the  flame  yellow,  indicating  soda. 
Arfvedsonite  fuses  easily  (3.5),  colors  the  flame  strong, 
persistent  yellow,  intumesces  decidedly  (difference  from 
aegirite)  and  yields  a  black,  shining,  magnetic  bead.  The 


ROCK-MAKING  MINERALS  65 

amphiboles  are  only  slightly  attacked  by  the  ordinary 
acids,  those  rich  in  iron  more  than  those  without. 

Alteration.  The  amphiboles  have  methods  of  alteration 
similar  to  those  of  the  pyroxenes.  Under  the  action  of 
various  agencies  they  may  be  changed  into  serpentine  or 
into  chlorite  or  into  both,  accompanied  by  the  formation 
of  carbonates,  sometimes  of  epidote  and  also  quartz. 
Under  the  continued  action  of  weathering  they  may 
break  down  further  in  limonite,  carbonates  and  quartz. 
Thus  on  much  weathered  rock  surfaces  only  rusty-looking 
holes  and  spots  may  be  left  to  show  their  former  presence. 

Occurrence.  Amphiboles  are  common  and  widely 
distributed  minerals  playing  an  important  role  in  igneous 
rocks  and  especially  in  the  metamorphic  ones.  The 
presence  of  water,  hydroxyl  and  fluorine  in  them  shows 
that  they  are  not  formed  by  simple  reactions  like  the 
pyroxenes  but  require  the  presence  of  mineralizing 
vapors;  they  are  in  some  sense  pneumatolytic  minerals. 
Thus  they  cannot  be  artificially  formed  by  allowing  simple 
dry  fusions  containing  their  constituent  silica  and  metallic 
oxides  to  cool  and  crystallize;  pyroxenes  are  produced 
instead  of  them.  And  if  hornblendes  are  fused  and  the 
melt  allowed  to  crystallize  we  obtain  pyroxenes,  iron  ore, 
etc.,  in  their  place;  this  is  because  the  necessary  water  and 
fluorine  have  escaped. 

Tremolite  is  chiefly  found  in  the  impure  crystalline  limestones  and 
dolomites  in  the  older  schistose  metamorphic  rocks  and  in  contact 
zones.  In  such  occurrences  it  not  infrequently  has  an  extra- 
ordinarily fine  fibrous  structure  and  is  capable  of  being  split  into 
long,  flexible  fibers  of  great  fineness  and  strength,  forming  the 
greater  part  of  what  is  known  as  asbestus.  Sometimes  actinolite 
and  other  hornblendes  are  found  in  this  asbestus  form.  Some  so- 
called  asbestus  is  really  a  fibrous  variety  of  serpentine. 

Actinolite  has  its  true  home  in  the  crystalline  schists;  it  is  the 
characteristic  light  green  to  bright  green  amphibole  of  many  horn- 
blende-schists and  greenstones :  in  many  of  these  cases  it  is  second- 
ary after  original  pyroxene  of  former  gabbro  and  trap  rocks  as 
described  under  uralite. 

Common  hornblende  occurs  both  in  igneous  and  metamorphic 


66         ROCKS  AND  ROCK  MINERALS 

rocks.  It  is  found  in  granites,  common  syenites,  and  in  the  doleritic 
types;  is  in  diorite  and  some  varieties  of  peridotite.  It  may  also  be 
often  observed  in  the  phenocrysts  of  felsitic  intrusive  porphyries  and 
lavas.  In  dark  traps  and  basalt  lavas  it  is  rare.  In  the  meta- 
morphic  rocks  it  is  found  in  gneisses  and  is  the  prominent  mineral  of 
the  hornblende  schists. 

Arjvedsonite  occurs  in  nephelite  syenites  and  in  rare  porphyries. 

Uralite  is  a  fibrous  or  fine  needle-like,  columnar  hornblende, 
secondary  after  pyroxene  and  as  mentioned  under  that  mineral 
produced  from  it  by  metamorphic  processes.  Instances  have  been 
found  where  the  outward  crystal  form  of  the  pyroxene  is  retained 
but  the  substance  composing  it  is  this  hornblende  in  parallel  bundles 
of  needle-like  prisms.  Generally  it  is  in  aggregates,  which  may  be 
very  fine  and  felt-like,  lying  in  the  plane  of  schistosity.  It  is  espe- 
cially apt  to  occur  when  basic,  pyroxenic,  igneous  rocks  have  been 
subjected  to  dynamic  changes  in  the  earth's  crust  attended  with 
squeezing  and  shearing.  It  varies  in  composition  from  actinolite 
to  common  hornblende,  depending  on  the  kind  of  pyroxene  from 
which  it  was  derived.  It  is  clear  that  it  cannot  be  a  simple  rearrange- 
ment of  the  pyroxene  molecule  since  the  latter  has  twice  as  much 
lime  as  the  hornblende  and  is  lacking  in  the  necessary  water  or 
fluorine.  Lime  is  separated  out  in  the  process  to  form  a  carbonate 
(calcite)  or  some  other  mineral  and  the  presence  of  water,  containing 
often  other  substances  in  solution,  is  a  necessary  aid  to  the  dynamic 
processes  of  pressure  and  shearing  which  set  up  chemical  activity 
and  the  reactions  which  produce  this  mineral. 

In  this  connection  the  reader  should  consult  what  is  said  under 
metamorphism  and  the  hornblende-schists. 

Determination.  Amphibole  may  be  confused  in  mega- 
scopic work  with  pyroxene,  tourmaline  and  epidote.  To 
distinguish  it  from  the  last  two  use  may  be  made  of  the 
various  physical  properties  mentioned  under  the  deter- 
mination of  pyroxene;  the  good  cleavage  separates  it  at 
once  from  tourmaline.  The  distinction  from  pyroxene 
is  much  more  difficult,  owing  to  the  fact  that  these  two 
minerals  have  similar  chemical  compositions  and  physical 
properties.  The  following  points  will  be  found  of  service 
in  this  connection.  If  the  mineral  appears  in  tolerably 
distinct  crystals  the  form  should  be  carefully  studied, 
especially  the  outline  of  the  section  of  the  prism  which 
can  often  be  observed  on  a  fractured  surface  of  the  rock 
and  comparison  made  with  Figs.  23  and  28. 


ROCK-MAKING  MINERALS  67 

In  case  the  crystal  form  is  imperfect  or  wanting,  if  it  is 
possible,  the  angle  at  which  the  cleavage  surfaces  meet 
should  be  carefully  studied,  as  this  is  a  fundamental 
character,  the  cleavage  prism  as  already  described  being 
nearly  square  in  pyroxene  and  much  more  oblique  in 
amphibole.  Further,  the  perfection  of  the  cleavage  in 
amphibole  and  the  bright  glittering  surfaces  it  yields 
furnish  indications  not  commonly  seen  in  pyroxene  whose 
cleavage  is  only  fairly  good.  Amphibole  also  is  apt  to 
occur  in  needles  or  long  bladed  prisms;  pyroxene  is  com- 
monly in  short  prismoids  or  grains.  Before  the  blow- 
pipe amphibole,  on  account  of  the  combined  water 
(hydroxyl),  is  more  apt  to  intumesce  than  pyroxene 
(arfvedsonite  from  aegirite)  but  this  cannot  be  relied  on  as 
a  general  definite  test.  If  fluorine  is  obtained  by  a 
qualitative  test  this  is  also  indicative  of  amphibole,  but 
many  do  not  contain  this  element  and  it  is  not  a  method 
which  is  ordinarily  in  one's  power  to  make.  Finally,  in 
many  cases,  especially  in  fine  grained  igneous  rocks,  it  is 
impossible  by  purely  megascopic  means  to  tell  if  the  dark 
ferromagnesian  mineral  present  is  hornblende  or  pyroxene 
or,  as  often  happens,  a  mixture  of  both.  Only  in  a  thin 
section  under  the  microscope  can  this  be  certainly  deter- 
mined. This  is  a  limitation  which  the  megascopic  method 
for  the  study  and  determination  of  rocks  and  rock-minerals 
imposes. 

OLIVINE. 

Form.  Olivine  crystallizes  in  the  orthorhombic  system  ; 
the  crystals  are  rather  complex  as  illustrated  by  a  common 
form  shown  in  Fig.  29.  The  form  is  not,  however,  a  matter 
of  importance,  as  the  mineral  very  rarely  shows  well 
developed  crystals  in  rocks  but  occurs  in  grains  or  small 
formless  masses  composed  of  grains. 

General  Properties.  There  is  a  cleavage  parallel  to  the 
face  b  but  it  is  not  a  very  perfect  or  noticeable  property 
in  rock  grains.  The  fracture  is  conchoidal.  The  color  is 


68 


ROCKS  AND  ROCK  MINERALS 


green,  generally  of  a  medium  shade  and  varying  from 
olive-green  to  a  yellow-green;  a  bottle-green  is  very 
common.  It  is  often  transparent  varying 
to  translucent  but  becomes  brown  to  dark 
red  on  oxidation  of  the  iron  and  more  or 
less  opaque;  this  is  frequently  noticed  in 
lavas  which  have  been  exposed  to  the  action 
of  steam.  Luster  vitreous;  streak  white 
to  yellowish.  Hardness  6.5-7.0.  Specific 
gravity  varies  with  the  iron  from  3.3-3.5. 
Chemical  Composition.  Olivine  is  magnesium  ortho- 
silicate,  Mg2Si04,  and  ferrous  orthosilicate,  Fe2SiO4,  which 
mingle  isomorphously  in  all  proportions.  The  nearly 
pure  magnesium  compound  is  called  forsterite,  the  nearly 
pure  iron  compound  faijalite',  these  occur  in  rocks  but  are 
rare.  Much  more  common  are  variable  mixtures  of  the 
two  which  make  common  olivine  or  chrysolite  as  it  is 
often  called.  These  variations  may  be  seen  in  the  fol- 
lowing table  of  analyses. 


Fig.  29 


SiO2 

MgO 

FeO 

XyO* 

Total. 

I  . 

41.8 

56.2 

1.1 

0.7 

99.8 

II  .... 
Ill  .... 

IV  .... 
V  

39.9 
37.2 
41.9 
33.6 

49.2 
39.7 
28.5 
16.7 

10.5 
22.5 
29.2 
44.4 

5.0 

99.6 
99.4 
99.6 
99.7 

VI  .... 

30.1 

68.2 

1.5 

99.8 

*  XyO  =  small  quantities  of  other  oxides,  chiefly  MnO. 

I,  Forsterite,  Monte  Somma,  Italy;  II,  Olivine,  Mt.  Vesuvius, 
Italy;  III,  Olivine,  Montarville,  Canada;  IV,  Olivine,  Hochbohl, 
Germany;  V,  Hortonolite,  Monroe,  Orange  Co.,  N.  Y.;  VI,  Fayalite, 
Rockport,  Mass. 

Blowpipe  and  Chemical  Characters.  Before  the  blow- 
pipe nearly  infusible;  varieties  very  rich  in  iron  fuse  and 
yield  magnetic  globules  —  these  are  apt  to  turn  red  on 
heating.  The  powdered  mineral  dissolves  in  hydrochloric 


ROCK-MAKING  MINERALS  69 

or  nitric  acid,  yielding  gelatinous  silica  on  evaporation. 
The  solution  may  be  tested  for  iron  and  magnesium  as 
directed  under  mineral  tests. 

Alteration.  In  one  case  this  takes  place  through  oxida- 
tion of  the  iron,  the  mineral  turns  reddish  or  brownish, 
and  eventually  a  mass  of  limonite  replaces  it,  accompanied 
with  carbonates  and  some  form  of  silica.  The  rusty  iron 
product  is  the  most  noticeable  feature  of  the  process. 

A  most  important  mode  of  alteration  is  that  by  which 
the  olivine  becomes  converted  into  serpentine.  This 
appears  to  take  place  through  the  agents  of  weathering 
near  the  surface  and  deeper  down  through  the  action  of 
heated  waters.  This  is  more  fully  discussed  under  the 
head  of  serpentine.  Other  substances  such  as  carbonate 
of  magnesia,  iron  ores,  free  silica,  etc.,  are  also  liable  to 
occur  as  by-products  in  the  process.  Other  kinds  of 
alteration  of  olivine  are  known  but  are  of  less  importance 
in  this  connection. 

Occurrence.  Olivine  is  a  quite  characteristic  mineral  of 
igneous  rocks,  especially  the  ferromagnesian  ones.  It  so 
rarely  occurs  in  those  composed  chiefly  of  alkalic  feldspars 
—  in  the  granite-syenite  rocks,  feldspathic  porphyries 
and  felsite  lavas  —  that  for  practical  purposes  it  need  not 
be  sought  in  them.  Anorthosite  is  the  only  feldspathic 
rock  in  which  it  may  become  of  importance.  Thus  its 
true  home  is  in  the  gabbros,  peridotites  and  basaltic 
lavas.  In  the  later  it  usually  occurs  in  bottle-green  grains; 
in  the  former  it  is  sometimes  colored  dark  by  inclusions. 
It  also  forms  masses  of  igneous  rock  known  as  dunite 
which  consist  almost  wholly  of  olivine.  Fine  trans- 
parent crystals  of  olivine  from  basaltic  lavas  are  fre- 
quently cut  for  gems,  commonly  called  peridotes.  The 
mineral  is  also  often  found  in  meteorites. 

Olivine  also  occurs  in  metamorphic  rocks,  in  crystalline  lime- 
stones of  dolomitic  character  and  in  other  rocks  found  in  such 
associations,  composed  of  varying  quantities  of  other  magnesian 
(and  lime)  silicates,  such  as  amphibole,  pyroxene  and  talc.  Its 


70 


ROCKS  AND  ROCK  MINERALS 


origin  may  be  ascribed  to  a  reaction  between  the  magnesium  car- 
bonate of  the  dolomite  and  quartz  sand  or  silica-bearing  solutions. 

2  MgCO3  +  Si02  =  Mg2SiO4  +  2  C02 

But  in  many  such  cases  of  its  occurrence  in  the  crystalline  schists. 
mixed  more  or  less  with  other  silicate  minerals,  its  presence  is  prob- 
ably due  to  the  fact  that  the  masses  containing  it  were  originally  of 
igneous  origin,  rather  than  metamorphosed  sedimentary  beds. 

Determination.  The  appearance,  associations  and 
characters  described  above  are  usually  sufficient  to  readily 
identify  the  mineral.  It  may  be  confused  with  greenish, 
more  or  less  transparent  grains  of  pyroxene,  but  the  lack  of 
pronounced  cleavage,  the  superior  hardness  and  easy 
gelatinization  in  acid  enable  one  to  distinguish  it  from  that 
mineral. 

GARNET. 

Form.  Garnets  crystallize  in  the  isometric  system  in 
the  simple  form  of  the  rhombic  dodecahedron  shown  in 

Fig.  30  or  in  the  trapezo- 

hedron  shown  in  Fig.  31. 

Sometimes,  they   show 

these  forms  well  developed 

and    are    then    excellent 

crystals,   which    may   be 

more  complicated  by  bev- 

ellings  or  truncations  of 
the  edges  of  the  dodecahedron.  Very  commonly  how- 
ever the  faces  are  not  well  developed  and  the  mineral 
then  appears  as  a  spherical  mass  or  grain. 

Cleavage  and  Fracture.  The  cleavage  is  generally  poor 
and  not  a  prominent  feature;  sometimes  a  parting,  in 
garnets  occurring  in  sheared  rocks,  may  be  seen  which 
suggests  a  lamellar  structure.  The  fracture  is  uneven. 
The  mineral  is  very  brittle  but  some  rocks  composed 
largely  of  massive  garnet  are  very  tough. 

Hardness  and  Specific  Gravity.  The  hardness  varies 
from  6.5-7.5;  the  specific  gravity  from  3.55  in  grossularite 
to  4.2  in  almandite,  common  garnet  being  about  4.0. 


Fig.  30 


Fig.  31 


ROCK-MAKING  MINERALS 


71 


Color,  Luster  and  Streak.  The  color  depends  upon  the 
composition;  grossularite  is  sometimes  white  but  usually 
tinted  pale  tones  of  green,  pink  or  yellow,  sometimes 
yellowish  or  reddish-brown  to  brown;  pyrope  is  deep  red 
to  black;  almandite  and  most  common  garnet  is  deep  red 
to  brownish-red;  melanite  is  black.  Streak,  light-colored, 
not  important.  The  luster  is  glassy,  sometimes  rather 
resinous.  The  light-colored  garnets  are  transparent  to 
translucent,  the  darker  ones  translucent  or  opaque. 

Chemical  Composition.  Garnets  are  orthosilicates  of 
the  general  formula  R'sRgCSiO^a,  in  which  the  radical  R 
may  be  calcium,  magnesium,  ferrous  iron  and  other 
bivalent  metals,  while  R  may  be  aluminum,  ferric  iron  or 
chromium,  trivalent  elements.  There  is  therefore  oppor- 
tunity for  a  number  of  combinations  which  are  isomor- 
phous.  The  most  common  ones  which  are  of  importance 
as  rock  minerals  are  grossularite ,  CasA^CSiO^s,  pyrope, 
,  almandite,  FesA^SiO^s  and  andradite, 
These  compounds,  however,  rarely,  if 
ever,  occur  pure,  generally  there  are  variable  amounts  of 
the  other  molecules  present  and  the  mineral  is  named 
from  the  one  predominating.  Common  garnet  is  chiefly 
almandite  with  more  or  less  of  the  others  present,  especially 
the  andradite  molecule,  and  at  times  this  may  predom- 
inate. Melanite,  the  black  garnet  found  in  some  rocks, 
is  chiefly  andradite.  These  facts  are  illustrated  in  the 
following  analyses  of  typical  specimens. 


SiO2 

A1203 

Fe203 

FeO 

MgO 

CaO 

XyO 

Total. 

I 

39.8 

22.1 

1.1 

0.7 

36.3 

100.0 

II  .     . 

40.4 

19.7 

4.0 

6.9 

20.8 

5.8 

2.6 

100.2 

Ill 

39.3 

21.7 

30.8 

5.3 

2.0 

1.5 

100.6 

IV.       . 

35.9 

19.2 

4.9 

29.5 

3.7 

2.4 

4.8 

100.4 

V    .       . 

35.7 

0.1 

30.0 

1.2 

0.1 

32.3 

0.9 

100.3 

I,  Grossularite,  Hull,  Ontario.  II,  Pyrope,  Krems,  Bohemia, 
XyO  =  Cr2O3.  Ill,  Almandite,  Fort  Wrangell,  Alaska,  XyO  =  MnO. 
IV,  Common  Garnet  (mostly  almandite),  Shimerville,  Perm. 

XyO  =  MnO;  V,  Andradite,  Sisersk.  Ural  Mts. 


72         ROCKS  AND  ROCK  MINERALS 

Blowpipe  and  Chemical  Characters.  The  garnets  fuse 
readily  before  the  blowpipe  and  in  the  reducing  flame 
those  containing  much  iron  become  magnetic.  After 
fusion  and  grinding  of  the  bead  to  powder  they  dissolve 
in  hydrochloric  acid  with  gelatinization  on  boiling.  They 
are  slightly  attacked  by  acids,  andradite  quite  strongly. 
Give  little  or  no  water  by  heating  in  closed  glass  tube. 
Decomposed  by  fusion  with  sodium  carbonate. 

Alteration.  Garnets  change  into  other  substances, 
commonly  chlorite,  serpentine,  etc.,  and  those  containing 
iron  oxides  may  alter  into  rusty  spots  of  limonite  and 
other  products  of  weathering. 

Occurrence.  Common  garnet  is  a  widely  distributed 
mineral  as  an  accessory  component  of  metamorphic  and 
sometimes  igneous  rocks.  Its  most  striking  occurrence 
is  in  schists,  especially  in  many  mica-schists  though  it  is 
also  found  in  other  kinds,  in  many  hornblende-schists  and 
in  gneisses  for  example.  It  is  apt  to  occur  in  the  ferro- 
magnesian  igneous  rocks  which  have  been  squeezed  and 
sheared.  It  is  sometimes  seen  in  granite-pegmatites, 
rarely  in  granite  itself,  in  occasional  scattered  crystals.  It 
also  occurs  in  the  contact  zone  of  igneous  rocks  where 
mixed  beds  containing  clay,  calcareous  matter  and 
limonite  have  been  metamorphosed.  Pyrope,  which 
chiefly  furnishes  the  garnet  used  as  a  jewel,  is  an  acces- 
sory component  of  some  peridotites  and  the  serpentines 
derived  from  them.  Grossularite  is  especially  found  in  re- 
crystallized  limestone  beds  both  in  contact  and  regional 
metamorphism.  Melanite  occurs  mostly  in  certain 
igneous  rocks  and  is  not  an  important  megascopic 
mineral. 

Determination.  The  crystal  form  of  garnets,  the 
appearance,  color  and  hardness  are  generally  sufficient 
to  enable  one  to  easily  recognize  them  and  in  case  of 
doubt  the  blowpipe  tests  will  furnish  sufficient  confir- 
mation. 


ROCK-MAKING  MINERALS 


73 


Fig.  33 


EPIDOTE. 

Form.     Epidote  crystallizes  in  the  monoclinic  system, 
the  simplest  form  being  that  shown  in  Fig.  32,  the  crystals 
are   apt    to    be    more    complex    with 
other  faces.      Well-developed  crystals       >• 

usually  occur  only  in  druses  in  seams     V 

and  cavities  and  the  form  is  there-  \  r  \ 
fore  not  generally  a  character  which 
can  be  of  much  use  in  megascopic  rock 
determination.  Commonly  seen  in  bladed  prisms  extended 
in  the  direction  of  the  edge  ac  and  sometimes  passing 
into  slender,  needle-like  forms.  Often  in  bundles  or 
aggregates  of  prisms  or  needles.  Terminations  of  prisms 
often  rounded.  Also  occurs  in  spherical  and  angular 
grains  and  in  aggregates  of  such  grains. 

General  Properties.  The  cleavage  is  perfect  parallel  to 
c,  parallel  to  a  imperfect.  Fracture  uneven.  Brittle. 
Hardness  is  6-7.  Specific  gravity  is  3.3-3.5.  The 
color  in  general  is  green,  usually  of  a  peculiar  yellowish, 
oily  green;  varying  from  pistache-green  to  olive,  some- 
times very  dark  green;  rarely  brownish.  Luster  vitreous. 
Streak  whitish.  Translucent  to  opaque. 

Chemical  Composition.  Epidote  is  really  the  name  of  a 
group  of  complex  silicates,  salts  of  orthosilicic  acid  whose 
hydrogen  atoms  are  replaced  by  calcium  and  by  a  set  of 
isomorphous  radicals  composed  of  variable  amounts  of 
alumina,  ferric  iron  and  sometimes  other  oxides  and  of 
hydroxyl.  Of  these  only  common  rock-making  epidote 
is  described  in  this  section  and  its  formula  may  be  repre- 
sented as  being  mixtures  of  Ca2(AlOH)Al2(Si04)3  and 
Ca2(FeOH)Fe2(Si04)3.  The  composition  may  be  seen 
in  these  two  specimen  analyses. 


SiO2 

A120, 

Fe203 

FeO 

CaO 

H2O 

XyO 

Total. 

I 

37.8 

22.6 

14.0 

0.9 

23.3 

2.1 

100.7 

II  .... 

37.0 

25.8 

10.0 

1.3 

21.9 

3.0 

1.0 

100.0 

I,    Untersulzbach,    Pinzgau.    II,    Macon   Co.,    North    Carolina, 
XyO  =  MnO  and  MgO  =  0.5  each. 


74  ROCKS  AND  ROCK  MINERALS 

Blowpipe  and  Chemical  Characters.  Before  the  blow- 
pipe epidote  fuses  easily  with  intumescence  to  a  black 
slaggy  mass.  Intense  heating  in  the  closed  glass  tube 
causes  the  finely  powdered  mineral  to  give  off  water. 
Only  slightly  acted  on  by  hydrochloric  acid  but  after 
fusion  dissolves  and  gelatinizes.  Reacts  with  fluxes 
for  iron  and  decomposes  on  fusion  with  sodium  car- 
bonate. 

Occurrence.  Epidote  is  characteristic  as  a  product  of 
alteration  of  other  minerals.  It  appears  through  the 
weathering  of  igneous  rocks  which  contain  largely  original 
lime,  iron  and  alumina  silicates  and  is  then  usually  with 
chlorite.  When  igneous  rocks  of  this  character  also 
suffer  regional  metamorphism  epidote  is  apt  to  form. 
The  occurrences  in  which  it  appears  most  notable  from  the 
megascopic  view  point  are  those  in  which  mixed  sedimen- 
tary beds  containing  calcareous  matter,  with  sand  clay 
and  limonite  (impure  limestones)  are  subjected  either  to 
general  or  contact  metamorphism.  Then  epidote  is  apt 
to  be  formed,  usually  in  company  with  other  silicates, 
but  sometimes  so  extensively  as  to  form  masses  which 
consist  almost  entirely  of  this  mineral. 

Determination.  The  peculiar  yellow-green  color,  superior 
hardness,  perfect  cleavage  in  one  direction  only  and 
the  blowpipe  characters  described  above  generally  suf- 
fice to  distinguish  epidote  from  hornblende,  pyroxene 
and  possibly  tourmaline  with  which  it  might  be  con- 
fused. The  hardness  distinguishes  it  at  once  from 
some  varieties  of  serpentine  which  resemble  it  in  color. 
This  may  be  confirmed  by  a  chemical  test  showing  the 
absence  of  magnesia  as  described  in  the  section  on 
mineral  testing. 

Zoisite.  This  is  a  mineral  which  has  the  same  chemical 
composition  as  epidote  and  is  closely  related  to  it.  It 
consists  almost  wholly  of  the  lime-alumina  molecule  pre- 
viously mentioned  and  contains  little  or  no  iron  oxides. 
It  is  orthorhombic  in  crystallization  but  in  the  crystals 


ROCK-MAKING  MINERALS  75 

seen  in  rocks  this  can  generally  only  be  told  by  optical 
methods :  it  occurs  in  aggregated  blades  or  prisms,  parallel 
or  divergent  or  in  grains  and  masses.  Its  color  is  usually 
gray  of  varying  shades.  From  epidotes  lacking  in  iron  it 
can  only  be  told  by  crystallographic  investigations.  » 

VESUVIANITE. 

Vesuvianite  is  a  tetragonal  mineral  which  generally 
crystallizes  in  short  thick  square  prisms  terminated  by  a 
pyramid  commonly  cut  off  by  a  basal  plane 
as  illustrated  in  Fig.  33.  It  also  occurs 
in  lumps  or  grains.  The  cleavage  is  poor, 
best  parallel  to  the  prism  faces  m;  fracture 
uneven.  The  color  generally  varies  from 
green  to  brown.  The  luster  is  vitreous. 
Hardness,  6.5.  Specific  gravity  about  3.4. 
Subtransparent  to  subtranslucent.  The 
chemical  composition  of  vesuvianite  is  not  exactly  known; 
it  is  a  silicate  of  calcium  and  aluminum  containing  hy- 
droxyl  and  fluorine,  but  small  amounts  of  ferric  iron  and 
magnesium  are  generally  present.  The  formula  has  been 
written  H4Ca12(AlFe)6Si10043  but  this  is  probably  not 
correct.  Before  the  blowpipe  it  fuses  readily  with  intu- 
mescence to  a  greenish  or  brownish  glass,  which  gelatinizes 
with  hydrochloric  acid.  The  fresh  mineral  is  slightly 
soluble  in  hydrochloric  acid. 

As  a  rock  forming  mineral  vesuvianite  characteristically 
occurs  in  limestones  which  have  become  crystalline 
through  the  contact  action  of  igneous  rocks  and  its  forma- 
tion is  evidently  conditioned  by  the  pneumatolytic 
emanations  of  water  and  fluorine  from  the  igneous  mag- 
mas. In  these  occurrences  it  is  commonly  associated 
with  garnet,  pyroxene,  tourmaline,  chondrodite,  and  other 
contact  minerals. 

Vesuvianite  may  be  confused  with  garnet,  pyroxene, 
epidote  or  hornblende,  but  the  study  of  its  crystal  form, 


76 


ROCKS  AND  ROCK  MINERALS 


its  other  physical  characters  and  behavior  before  the  blow- 
pipe will  generally  serve  to  distinguish  it  from  them. 

STAUROLITE. 

Form.  Staurolite  is  orthorhombic  in  crystallization 
and  usually  in  distinct  crystals  of  the  form  shown  in  Fig. 
34.  They  are  often  stout  and  thick,  sometimes  long  and 
more  slender  but  not  strikingly  so.  The  angle  of  the 


Fig.  34 


Fig.  35 


Fig.  36 


faces  m  on  m  is  50°  40'.  They  are  terminated  by  flat 
bases  c,  though  it  often  happens  these  cannot  be  seen 
in  the  rock.  Staurolite  is  very  apt  to  form  compound 
twinned  crystals  as  shown  in  Figs.  35,  36.  From  this 
fact  its  name  is  derived  from  the  Greek,  meaning  a  cross. 

Physical  Properties.  The  mineral  has  a  moderate  but 
distinct  cleavage  parallel  to  the  face  b;  the  fracture  is  sub- 
conchoidal.  The  color  varies  from  a  dark  reddish  or 
yellowish  brown  to  almost  black,  the  light  transmitted 
through  thin  splinters  appears  almost  blood-red.  The 
streak  is  white  to  gray.  The  hardness  is  7-7.5,  the  specific 
gravity  3.75. 

Chemical  Composition.  The  formula  is  rather  complex, 
(AlO)4(A10H)Fe(Si04)2,  the  alumina  may  be  partly 
replaced  by  ferric  iron  and  the  ferrous  iron  by  magnesia, 
as  seen  in  the  included  analysis  of  a  crystal  from  Franklin, 
North  Carolina.  The  percentage  of  silica  is  very  low; 

Si02    A1203    Fe203    FeO    MgO    H2O  Total 
27.91    52.92      6.87     7.80    3.28     1.59  =  100.37 


ROCK-MAKING  MINERALS  77 

staurolite  is  one  of  the  rock-forming  silicates  containing 
the  least  silica  and  this  fact  with  the  high  alumina  is 
significant  of  its  place  and  mode  of  origin  —  in  metamor- 
phosed clay  rocks. 

Blowpipe  and  Chemical  Characters.  Staurolite  is  prac- 
tically infusible  before  the  blowpipe.  It  is  almost  in- 
soluble in  acids.  It  may  be  fused  with  carbonate  of  soda 
and  the  resulting  fusion  after  solution  in  hydrochloric 
acid  may  be  tested  for  alumina,  iron  and  magnesia.  It  is 
easily  recognized  by  its  color,  crystal  form,  hardness, 
method  of  twinning  and  mode  of  occurrence. 

Occurrence.  Staurolite  occurs  in  the  metamorphic 
rocks;  it  is  a  highly  characteristic  mineral  of  the  crystal- 
line schists.  It  is  found  in  mica  schists,  in  certain  slates 
and  sometimes  in  gneiss.  Frequently  it  is  associated 
with  dark  red  garnets  in  these  rocks. 

ANDALUSITE. 

Andalusite  is  orthorhombic  in  crystallization  and  is  usually  seen 
in  rough  prisms,  nearly  square  in  cross  section.  Sometimes  the 
prisms  are  collected  in  radiated  groups.  The  cleavage  parallel  to 
the  prism  is  good  —  in  other  directions  poor.  Fracture  uneven  to 
subconchoidal.  The  normal  color  is  white  to  pink  or  red  to  brown, 
but  the  mineral  is  very  apt  to  contain  impurities,  especially  particles 
of  carbonaceous  matter,  which  may  color  it  dark  or  even  black. 
Often  these  are  arranged  in  a  symmetrical  manner  in  the  crystal  so 
that  the  cross  section,  when  it  is  broken  or  cut,  displays  a  definite 
pattern,  such  as  a  white  cross  in  a  black  square.  This  may  help  to 
identify  the  mineral.  It  is  usually  subtranslucent  in  thin  splinters. 
Brittle.  Hardness,  7.5.  Specific  gravity,  3.2.  Streak,  whitish. 
The  chemical  composition  is  Al2SiO5  =  A12O3 .  SiO2.  It  is  insoluble 
in  acids  but  decomposed  by  fusion  with  carbonate  of  soda.  Before 
the  blowpipe  it  is  infusible;  after  moistening  with  cobalt  nitrate 
solution  it  turns  a  blue  color  upon  intense  ignition  (as  do  also  cyanite 
and  some  other  alumina  minerals). 

Andalusite  is  a  mineral  characteristic  of  metamorphism,  and  es- 
pecially of  the  contact  zones  of  igneous  rocks,  such  as  granite.  It 
is  produced  by  the  alteration  of  clay  slates  and  shales  as  described 
on  a  later  page.  It  occurs  in  mica  schists  and  gneisses;  sometimes 
though  rarely  it  is  found  in  granite. 


78  ROCKS  AND  ROCK  MINERALS 

CYANITE. 

Cyanite  usually  occurs  in  long  bladod  crystals  which  rarely  show 
distinct  end  faces,  or  in  coarsely  bladed  columnar  masses.  It  is 
triclinic.  It  has  one  very  perfect  cleavage  (parallel  to  the  face  a) 
and  another  less  so  (parallel  to  6);  the  angle  between  these  is  about 
74  degrees.  The  color  is  white  to  pure  blue,  sometimes  the  center 
of  the  blade  is  blue  with  white  margins;  rarely  gray,  green  to  black. 
Streak  whitish.  Transparent  to  translucent.  Luster  vitreous  to 
pearly.  Hardness  varies  in  different  directions  from  5-7;  least  on 
face  a  (best  cleavage)  greatest  on  face  6  (second  cleavage).  Specific 
gravity,  3.56-3.67.  Chemical  composition,  Al2SiO5,  and  other 
chemical  and  blowpipe  properties  similar  to  those  of  andalusite, 
mentioned  above. 

Cyanite  is  a  mineral  characteristically  developed  in  regions  sub- 
jected to  intense  regional  metamorphism.  It  occurs  in  gneisses  and 
in  mica  schists.  In  the  latter  case  the  mica  is  sometimes  muscovite 
and  sometimes  the  soda-bearing  variety,  paragonite.  It  is  often 
associated  with  garnet,  sometimes  with  staurolite  or  corundum.  It 
alters  to  talc  and  steatite. 

Cyanite  is  easily  distinguished  from  other  minerals,  especially 
andalusite  which  has  the  same  chemical  composition  by  its  form, 
color,  variable  hardness,  specific  gravity  and  other  properties. 

Sillimanite.  With  andalusite  and  cyanite  may  be  mentioned 
sillimanite,  a  mineral  which  has  a  chemical  composition  identical 
with  them,  but  which  is  separated  mineralogically  because  it  has  a 
different  crystal  form  as  shown  by  its  angles.  Its  chief  importance 
is  microscopical,  but  it  may  sometimes  be  seen  with  the  eye  or  lens, 
mostly  in  gneisses  or  quartzites,  as  slender  white  or  light-colored, 
four-sided  prisms,  or  radiated  aggregates  of  them  forming  brushes. 
Its  blowpipe  and  other  chemical  characters  are  like  those  described 
for  andalusite  and  cyanite. 

TOURMALINE. 

Tourmaline  is  a  mineral  of  which  there  are  a  number  of 
varieties  based  on  the  color,  which  in  turn  depends  on  the 
chemical  composition.  The  chief  ones  are  black,  green, 
brown  and  red,  but  of  these  the  black  variety  known  also  as 
schorl  is  the  only  one  which  is  of  importance  in  mega- 
scopic petrography. 

Form.  Tourmaline  crystallizes  in  the  rhombohedral 
division  of  the  hexagonal  system.  The  faces  therefore 


ROCK-MAKING  MINERALS 


79 


are  in  threes  or  multiples  of  three.  A  simple  form  is 
shown  in  Fig.  37  and  its  appearance  looking  down  upon 
the  upper  end  in  Fig.  39.  It  consists  of  the 
three-cornered  prism  m  its  edges  bevelled 
by  the  prism  faces  a  and  terminated  by 
the  rhombohedron  r.  The  crystals  if  well 
developed  are  apt  to  be  more  complicated 
than  this,  other  faces  being  present  and  if 
both  ends  are  perfect  they  have  unlike  faces. 
Though  sometimes  short  and  thick  the  crys- 
tals are  commonly  elongated  prisms,  often 
extremely  long  and  thin.  Very  often  also 
the  faces  a  and  m  oscillate  or  repeat  so 
that  the  prism  is  striated  or  channeled  as 
shown  in  Fig.  38  and  the  outline  and  appear- 
ance from  above  is  that  seen  in  Fig.  40.  This 
spherical  triangle  cross  section  is  very  characteristic  of  the 
prisms  of  rock-making  tourmaline.  It  is  rarely  in  form- 


Fig- 37 


Fig.  40 


less  grains  or  large  shapeless  masses.  The  slender  prisms 
and  needles  are  apt  to  be  aggregated  together  into  bundles, 
sheaves  and  radiate  groups.  The  section  of  the  latter 
in  rocks  furnishes  the  so-called  "  tourmaline  suns." 

General  Properties.  Tourmaline  has  no  good  cleavage 
and  its  fracture  is  rather  conchoidal  to  uneven.  It  is 
brittle.  The  color  is  black,  the  luster  glassy,  sometimes 
dull,  streak  uncolored,  not  characteristic.  Opaque.  The 


80 


ROCKS  AND   ROCK  MINERALS 


hardness  is  7—7.5,  the  specific  gravity  3.1—3.2.     It  becomes 
electrified  by  friction. 

Chemical  Composition.  Tourmaline  is  a  very  complex 
silicate  of  boron  and  aluminium  with  hydroxyl  and  some- 
times fluorine  and  with  magnesium,  iron  and  sometimes 
alkali  metals.  It  may  be  said  to  be  a  salt  of  an  alumin- 
ium-borosilicic  acid  in  which  the  hydrogens  are  re- 
placed by  iron,  magnesium,  alkalies  and  aluminium  in 
varying  amounts.  This  acid  has  been  formulated  as 
H8Al3(BOH)2Si4O13  and  in  common  black  tourmaline 
the  hydrogens  are  replaced  mostly  by  iron  or  iron  and 
magnesia  as  shown  in  the  analyses  here  given. 


Si02 

A1203 

Fe203 

FeO 

MgO 

Na2O 

B203 

H2O 

XyO 

Total. 

I. 

35.0 

34.4 

1.1 

12.1 

1.8 

2.0 

9.0 

3.7 

0.6 

99.7 

II  . 

35.6 

25.3 

0.4 

8.2 

11.1 

1.5 

10.1 

3.3 

4.3 

99.8 

I,    Paris,    Maine.     II,    Pierrepont, 
quantities  of  other  oxides,  etc. 


New    York.     XyO  =  small 


Blowpipe  and  Chemical  Characters.  Difficultly  fusible 
before  the  blowpipe  with  swelling  and  bubbling.  When 
mixed  with  powdered  fluor  spar  and  bisulphate  of  potas- 
sium momentarily  colors  the  flame  a  fine  green,  showing 
presence  of  boron.  Decomposed  on  fusion  with  sodium 
carbonate.  Not  acted  on  by  acids  but  after  fusion  gelat- 
inizes in  hydrochloric  acid. 

Occurrence.  Tourmaline  is  not  a  common  megascopic 
component  of  rocks,  but  it  is  of  interest  and  importance 
because  it  is  perhaps  the  most  common  and  typical 
mineral  which  is  produced  in  the  pneumatolytic  or 
fumarole  stage  of  igneous  rock  formation  as  described  in 
another  place.  This  is  shown  by  the  boron,  hydroxyl  and 
fluorine  which  it  contains.  Thus  it  is  one  of  the  most 
common  and  characteristic  accessory  minerals  found  in  the 
pegmatite  dikes  associated  with  intrusions  of  granites;  its 


ROCK-MAKING  MINERALS 


81 


presence  in  granite  indicates,  as  a  rule,  nearness  to  the 
contact  and  in  the  rocks  which  have  suffered  contact 
metamorphism  it  is  very  liable  to  appear.  In  this  way 
it  is  not  infrequently  found  associated  with  certain  ore 
deposits.  It  appears  at  times  also  in  gneisses,  in  schists 
and  in  crystalline  limestones  of  the  metamorphic  rocks 
and  its  occurrence  in  these  cases  indicates  that  the  meta- 
morphism has  been  induced  in  part  by  the  contact  action 
of  igneous  masses  giving  off  water  vapors  and  other 
volatile  substances.  The  beautiful  red  and  green  trans- 
parent tourmalines  which  are  valued  as  gem  material 
occur  in  pegmatite  dikes  of  granite,  often  associated  with 
the  common  black  variety.  The  red  is  usually  found  with 
lepidolite  —  the  lithia  mica. 

Determination.  The  black  color,  crystalline  form,  and 
mode  of  occurrence  of  common  tourmaline  are  usually 
sufficient  to  identify  it.  From  black  hornblende  it  is 
easily  distinguished  by  its  lack  of  good  cleavage,  superior 
hardness  and  the  shape  of  the  cross  section  of  the  prism  and 
this  can  be  made  certain  by  the  blowpipe  test  for  boron. 

TOPAZ. 

Topaz  crystallizes  in  the  orthorhombic  system  and  the 
form  in  which  it  is  generally  seen  is  in  pointed  prisms,  as 
illustrated  in  Fig.  41.  There  is  a  very 
perfect  cleavage  parallel  to  the  base  c,  at 
right  angles  to  the  prism:  the  fracture  is 
uneven.  The  mineral  is  very  hard  =  8, 
and  brittle.  The  specific  gravity  is  about 
3.5.  In  color  it  is,  while  generally  trans- 
parent, often  colorless,  sometimes  yellow 
to  brown  -  yellow,  sometimes  white  and 
translucent.  The  luster  is  vitreous.  The 
chemical  composition  as  established  by 
Penfield  is  (AlF2)SiO4  in  which  the  fluorine  may  be 
replaced  in  part  by  hydroxyl  ( — OH).  Before  the 
blowpipe  it  is  infusible.  If  fused  in  a  closed  tube  with 


82         ROCKS  AND  ROCK  MINERALS 

previously  fused  and  powdered  phosphorus  salt  hydro- 
fluoric acid  will  be  given  off  which  etches  the  glass  and 
deposits  a  ring  of  silica  on  the  colder  upper  walls  of  the 
tube.  If  the  pulverized  mineral  be  moistened  with 
cobalt  nitrate  solution  and  intensely  heated  before  the 
blowpipe  on  charcoal  it  assumes  a  fine  blue  color  showing 
presence  of  alumina. 

Topaz,  while  not  a  common  or  important  rock-forming  mineral, 
is  a  very  interesting  one  as  it  is  particularly  characteristic  of  the 
pneumatolytic  stage  in  the  formation  of  igneous  rocks. 

Thus  it  is  found  in  crystals  in  the  miarolitic  cavities  of  granites 
where  the  vapors  have  collected  and  in  the  same  way  in  felsite  lavas 
(especially  in  rhyolite).  It  is  also  found  in  pegmatite  dikes  and  in 
the  cracks  and  crevices  of  the  surrounding  rocks  which  have  served 
as  channel  ways  for  the  escape  of  gases  as  explained  under  the 
description  of  pegmatite  dikes  and  of  contact  metamorphism.  It 
is  apt  to  be  associated  in  these  occurrences  with  quartz,  mica,  tour- 
maline and  sometimes  with  cassiterite,  tin  ore. 

The  form,  color,  cleavage  and  great  hardness  of  topaz,  together 
with  its  mode  of  occurrence,  serve  to  readily  distinguish  it  from 
other  minerals  and  the  determination  may  be  confirmed  by  the 
chemical  tests  mentioned  above. 

CHONDRODITE. 

Chondrodite  is  really  one  of  a  small  group  of  minerals  —  chon- 
drodite,  humite,  clinohumite,  etc.  —  which  are  so  closely  allied  in 
all  of  their  general  properties  that  for  practical  megascopic  rock 
work  they  are  indistinguishable  and  may  all  be  comprised  under 
this  heading.  While  the  mineral  is  monoclinic  it  rarely  shows,  as 
a  rock  component,  any  definite  crystal  form  which  is  of  value  in 
determining  it,  but  appears  as  embedded  grains  and  lumps.  The 
cleavage  is  not  marked  but  is  sometimes  distinct  in  one  direction. 
Brittle;  fracture  subconchoidal.  The  color  is  yellow,  honey  yellow 
to  reddish  yellow,  to  brown  red.  Luster  vitreous.  Hardness 
6-6.5.  Specific  gravity  3.1-3.2.  In  chemical  composition  the 
mineral  is  closely  allied  to  olivine  but  differs  in  containing  fluorine  or 
hydroxyl  or  both,  as  may  be  seen  from  the  following  formulas  deduced 
by  Penfield. 

Olivine  =  Mg2SiO4. 

Chondrodite  =  Mgj;Mg(F,OH)]2(SiO4)2. 
Humite  =  Mg^Mg(F,OH)]2(Si04)3. 
Clinohumite  =  Mg7[Mg(F,OH)]2(SiO4)4. 


ROCK-MAKING  MINERALS  83 

As  in  olivine  part  of  the  magnesium  is  usually  replaced  by  some 
ferrous  iron.  The  powdered  mineral  is  slowly  dissolved  by  hydro- 
chloric acid,  yielding  gelatinous  silica.  The  solution  evaporated  to 
dryness,  then  moistened  with  acid  and  taken  up  in  water,  after  the 
silica  is  filtered  off,  yields  tests  for  iron  with  excess  of  ammonia  and 
after  its  separation  by  filtering,  for  magnesia  with  sodium  phosphate 
solution.  From  olivine  it  may  be  distinguished  by  a  test  for  fluorine 
as  described  under  topaz.  Before  the  blowpipe  it  is  nearly  infusible. 

The  characteristic  mode  of  occurrence  of  chondrodite  is  in  lime- 
stone, especially  dolomite,  which  has  been  subjected  to  contact 
action  of  igneous  rocks.  In  them  it  forms  yellowish  or  reddish 
embedded  grains  or  lumps  usually  associated  with  other  contact 
minerals  such  as  pyroxene,  vesuvianite,  magnetite,  spinel,  phlo- 
gopite,  etc.  The  presence  of  the  hydroxyl  and  fluorine  shows  its 
derivation  by  pneumatolytic  processes. 

The  appearance,  color,  mode  of  occurrence  and  associations  are 
usually  sufficient  to  identify  the  mineral,  and  these  may  be  con- 
firmed by  the  chemical  tests  mentioned. 

6.     Oxides,  etc. 

The  list  of  important  rock-making  oxides  includes  first, 
silica,  Si02  and  then  corundum,  A12O3.  Then  come  the 
oxides  of  iron  which  are  of  importance  as  nearly  constant 
accessory  minerals  in  rocks  and  therefore  have  a  wide 
distribution.  For  this  reason  two  other  minerals  not 
oxides,  pyrite,  the  sulphide  of  iron,  and  apatite,  the  phos- 
phate of  lime,  are  also  here  included.  Limonite,  the 
hydrated  oxide  of  iron,  which  is  always  secondary,  is 
placed  with  the  other  iron  ores  for  the  sake  of  convenience. 

QUARTZ. 

Form.  Quartz  crystallizes  in  the  hexagonal  system, 
the  ordinary  form  being  a  hexagonal  prism  terminated  by 
a  six-sided  pyramid.  This  form,  which  is  the  common  one 
for  the  crystals  of  veins  and  is  illustrated  in  Fig.  42,  is  not 
often  seen  in  the  quartzes  of  rocks,  except  in  igneous  rocks 
which  possess  miarolitic  or  drusy  cavities;  into  them  the 
rock-making  quartzes  project  with  free  ends  which  show 
crystal  form.  The  large  crystals  seen  in  pegmatite  veins 
and  which  sometimes  attain  huge  dimensions  are  only  a 


84 


ROCKS  AND  ROCK  MINERALS 


Fig.  4* 


Fig.  43 


manifestation  of  the  same  thing  on  a  larger  scale,  as 
explained  under  the  pegmatite  formation  of  igneous  rocks. 
In  porphyries  where  quartz  may  have  crys- 
tallized free  as  phenocrysts  it  tends  to  take 
the  form  shown  in  Fig.  43;  the  two  pyramids 
are  present  and  the  prism  is  very 
short  or  even  wanting.  Since  the 
crystals  are  usually  poorly  devel- 
oped, with  rough  faces,  they  appear 
as  spherical  objects,  like  shot  or 
peas,  embedded  in  the  rock,  with 
round  cross  sections  where  broken 
across  on  a  fracture  face.  In  general,  quartz  has  no 
definite  form  in  rocks,  especially  in  igneous  ones  like 
granite,  where,  being  usually  the  last  substance  to  crys- 
tallize, its  shape  is  conditioned  by  the  other  minerals 
which  have  already  formed.  In  granites,  therefore, 
it  commonly  appears  in  small  shapeless  lumps  and 
masses,  but  in  some  of  the  fine-textured  varieties  the 
quartz  tends  to  appear  in  granules  like  those  composing 
lump  sugar.  In  pegmatite  dikes  it  appears  on  fracture 
surfaces  in  curious  script-like  figures  intergrown  with 
feldspar,  forming  the  substance  known  as  graphic  granite. 
Cleavage  and  Fracture.  The  cleavage  of  quartz  is  so 
poor  that  for  practical  petrographic  purposes  it  may  be 
regarded  as  not  possessing  any.  It  has  commonly  a  good 
conchoidal  fracture  which  is  a  great  help  in  distinguishing 
it  in  granitic  rocks  but  in  some  massive  forms  it  is  uneven 
and  splintery.  The  mineral  is  brittle  to  tough. 

Color  and  Luster.  Rock-making  quartz  varies  in  color 
from  white  through  shades  of  gray  and  dark  smoky  gray 
or  brown  to  black.  The  gray  and  smoky  tones  are  most 
common  in  igneous  rocks  and  the  white  color  in  the  sedi- 
mentary and  metamorphic  ones  but  there  is  no  absolute 
rule  about  this.  The  black  color  is  rare  and  mostly  con- 
fined to  igneous  rocks;  sometimes  in  them  it  has  a  strong 
bluish  tone.  The  colorless,  limpid  quartz,  so  characteristic 


ROCK-MAKING  MINERALS  85 

of  the  crystals  found  in  veins  and  geodes  and  deposited  by 
solution,  is  rare  as  a  rock-making  component  but  some- 
times occurs  as  in  some  very  fresh  lavas.  The  mineral 
may  also  at  times  possess  an  exotic  color  given  it  by  some 
substance  acting  as  a  pigment;  thus  it  may  be  red  from 
included  ferric  oxide  dust  or  green  from  scales  of  chlorite, 
and  in  the  sedimentary  and  metamorphic  rocks,  such  as 
quartzite,it  may  be  very  dark  from  included  organic  matter 
or  charcoal-like  substance. 

The  luster  varies  from  glassy  to  oily  or  greasy.  The 
streak  is  white  or  very  pale  colored  and  not  a  prominent 
character.  Hardness,  7.  Scratches  feldspar  and  glass 
but  is  not  touched  by  the  knife.  Specific  Gravity  =  2.66. 

Composition.  Pure  silica,  Si02.  This  is  the  composi- 
tion of  the  crystallized  common  rock-making  quartz,  but 
certain  massive  varieties  of  silica,  which  are  not  crystallized 
or  not  apparently  so,  and  are  of  common  occurrence  and 
sometimes  take  part  in  forming  rocks,  such  as  jasper, 
opal,  chert,  etc.,  contain  in  addition  more  or  less  com- 
bined water,  while  impurities  like  clay,  oxides  of  iron, 
etc.,  are  usually  present  and  give  them  distinctive  colors. 

Blowpipe  and  Chemical  Characters.  Quartz  is  infusible 
before  the  blowpipe  —  varieties  dark  from  organic  matter 
whiten  but  do  not  fuse.  Fused  with  carbonate  of  soda, 
it  dissolves  with  effervescence  of  C02  gas.  In  the  sodium 
metaphosphate  bead  a  fragment  floats  without  dissolv- 
ing. It  is  insoluble  in  acids  except  hydrofluoric,  HF. 

Occurrence.  Quartz  is  one  of  the  commonest  of  all 
minerals,  and  is  universally  distributed,  occurring  in 
igneous,  sedimentary  and  metamorphic  rocks  alike.  Not 
only  does  it  form  rocks  in  company  with  other  minerals, 
chiefly  feldspar,  but  in  pure  sandstones  and  quartzites 
it  may  be  the  only  one  present  in  the  rock-mass.  It  is 
indeed  so  common  that,  with  the  exception  of  the  lime- 
stones and  marbles  and  dark  heavy  igneous  rocks  like 
dolerite  and  basalt,  its  presence  in  rocks  should  at  least 
always  be  suspected. 


86  ROCKS  AND  ROCK  MINERALS 

Determination.  The  hardness  of  quartz,  its  lack  of 
cleavage,  its  conchoidal  fracture  and  generally  greasy 
luster  are  characters  which  help  to  distinguish  it,  especially 
from  the  feldspars  with  which  it  is  so  often  associated. 
The  gray  and  smoky  color  it  often  has  in  granites  and 
other  igneous  rocks  helps  in  the  same  way.  It  may  be 
confused  with  nephelite  but  this  mineral  is  readily  soluble 
in  acids  with  gelatinization  and  moreover  is  very  rare. 
These  characters,  with  the  blowpipe  and  chemical  ones 
mentioned  above,  will  readily  confirm  its  determination. 

Opal,  Jasper,  Flint,  Chalcedony,  etc.  Silica,  in  addition  to 
forming  the  crystallized  anhydrous  mineral  quartz,  occurs  in  non- 
crystalline,  amorphous  masses  which  contain  varying  amounts  of 
water.  Accordingly  as  the  color,  structure  and  other  properties 
vary,  a  great  number  of  different  varieties  are  produced  which  have 
received  particular  names.  For  a  description  of  them  the  larger 
manuals  of  mineralogy  should  be  consulted.  They  seem  to  have 
been  formed,  in  large  part  at  least,  by  the  evaporation  of  liquids 
containing  soluble  silica,  which  on  the  drying  down  has  been 
deposited  in  an  amorphous,  more  or  less  hydrated  condition  instead 
of  as  crystalline  quartz.  Sometimes  they  are  a  mixture  of  quartz 
particles  or  fibers  mixed  with  amorphous  material.  This  form  of 
silica  is  illustrated  by  the  gelatinous  product  obtained  when  a 
silicate  like  nephelite  is  dissolved  in  an  acid  and  the  resulting  solution 
evaporated.  It  is  also  formed  in  nature  as  a  secretion  from  water 
by  various  living  organisms. 

Amorphous  silica  is  not  a  rock  component  of  any  megascopic 
importance  in  igneous  or  metamorphic  rocks,  but  in  the  sedimentary 
ones  it  forms  accompanying  masses  and  sometimes  beds  which,  al- 
though not  of  wide  general  importance,  may  be  of  considerable  local 
interest  and  value.  These  are  further  noticed  in  their  appropriate 
places.  It  may  also  act  as  a  cementing  substance  of  the  grains  of 
some  rocks. 

CORUNDUM. 

Form.  The  crystallization  is  hexagonal  and  the  form 
assumed  is  either  a  thick  six-sided  prism  often  swelling  out 
in  the  middle  into  barrel-like  shape  or  in  thinner  six-sided 
tables;  also  commonly  in  grains  or  shapeless  lumps. 
The  thick  and  barrel  forms  are  most  common  when  it 


ROCK-MAKING  MINERALS  87 

occurs  in  massive  rocks  like  the  syenites,  and  they  are 
associated  with  the  grains  and  lumps.  Sometimes  on 
parting  faces  a  multiple  twinning  resembling  that  illus- 
trated as  occurring  on  feldspars  may  be  observed,  pro- 
duced however  by  another  method. 

Cleavage.  Corundum  does  not  have  a  good  cleavage 
but  possesses  a  parting  that  appears  like  perfect  cleavage 
parallel  to  the  base  of  the  prism  and  also  in  three  other 
directions  at  an  angle  to  it  (parallel  to  the  unit  rhombo- 
hedron).  In  large  pieces  these  partings  or  pseudo- 
cleavages  may  appear  nearly  at  right  angles  and  the 
mineral  has  a  laminated  structure. 

Color,  Luster,  and  Hardness.  Rock-making  corundum 
is  usually  dark  gray  to  bluish  gray  or  smoky.  It  is  very 
rarely  blue  forming  the  variety  sapphire,  while  the  red 
variety  or  ruby  is  excessively  rare.  The  luster  is  adaman- 
tine to  vitreous,  sometimes  dull  and  greasy  in  rock  grains. 
Translucent  to  opaque.  It  is  the  hardest  of  rock  minerals 
=  9.  Brittle,  though  sometimes  very  tough.  Specific 
Gravity  =  4. 

Blowpipe  and  Chemical  Characters.  Before  the  blow- 
pipe it  is  infusible.  The  powder  moistened  with  cobalt 
solution  and  intensely  ignited  turns  bright  blue  showing 
alumina.  It  is  insoluble  in  acids.  Its  composition  is 
pure  alumina,  A12O3.  By  the  action  of  weathering  and 
alteration  it  is  apt  to  change  into  muscovite. 

Occurrence.  In  recent  years  corundum  has  been 
recognized  as  an  important  primary  mineral  in  the  igneous 
rocks  of  a  number  of  regions,  in  syenites  in  Canada, 
Montana  and  India,  in  peridotites  in  North  and  South 
Carolina  and  Alabama,  and  in  other  igneous  rocks  in  the 
Urals  and  in  California.  An  unrecorded  occurrence  is  in 
syenite  in  Orange  County,  New  York  State.  Many  more 
such  will  doubtless  be  discovered.  The  variety  sapphire 
has  been  found  in  basaltic  rocks  in  Montana,  the  Rhine 
district  and  elsewhere.  Corundum  also  occurs  in  the 
contact  zone  of  igneous  rocks.  In  these  cases  it  is  usually 


88  ROCKS   AND  ROCK  MINERALS 

in  thin  tabular  crystals.  It  also  occurs  in  metamorphic 
rocks,  sometimes  in  thick  beds  of  the  variety  called 
emery.  Probably  in  many  of  these  occurrences  it  ante- 
dates the  period  of  metamorphism  and  is  of  igneous 
origin. 

Determination.  The  crystal  form,  when  present,  and 
color  indicate  the  presence  of  this  mineral  which  is  readily 
confirmed  by  a  test  of  its  hardness  since  it  cannot  be 
scratched  by  another  of  the  rock  minerals.  These  tests 
may  be  confirmed  by  the  other  described  properties. 

THE   IRON   ORES. 

The  term  ore  is  commonly  applied  to  the  oxides,  sul- 
phides and  carbonates  of  the  heavy  metals  as  the  sources 
from  which  they  are  obtained  in  commercial  quantities. 
Of  these  minerals  the  only  ones,  which  by  reason  of  their 
wide  distribution  and  common  occurrence  as  components 
of  rocks,  may  be  considered  of  general  importance  from 
the  petrological  standpoint  are  the  oxides  and  sulphides 
of  iron.  Even  these  play  only  a  subordinate  role  in 
rock-making  and  are  considered  as  accessory  minerals, 
except  in  certain  cases  where  they  have  been  concen- 
trated by  geologic  processes  into  considerable  masses. 
They  are  considered  accessory  because,  in  one  form  or 
another,  they  are  found  scattered  in  small  quantities 
through  most  rocks  and  in  each  of  the  three  great  classes 
of  rocks  and  do  not  therefore  have  the  same  importance 
and  value  in  classification  that  those  minerals,  such  as 
feldspars  and  pyroxenes  have,  which  occur  in  large  and 
varying  amounts.  They  are  mentioned  here  because  they 
are  the  most  common  of  accessory  rock-minerals  and  are 
of  importance  in  other  ways  as  well.  They  include 
magnetite,  ilmenite,  hematite,  limonite  and  pi/rite.  There 
are  other  oxides  and  sulphides  of  iron  but  they  are  rela- 
tively of  small  petrographic  importance. 


KOCK-MAKING  MINERALS 


89 


MAGNETITE. 


Form.  Magnetite  crystallizes  in  the  isometric  system, 
most  commonly  in  octohedrons,  Fig.  44,  sometimes  in 
dodecahedrons,  Fig.  45,  sometimes  in  a  combination  of 


Fig.  44 


Fig.  45 


Fig.  46 


both,  Fig.  46.  It  is  sometimes  seen  in  distinct  crystals 
in  rocks  but  usually  is  in  small  grains  whose  form  cannot 
be  made  out  and  is  sometimes  in  larger  irregular  masses. 

General  Properties.  No  distinct  cleavage  but  some- 
times a  parting  parallel  to  the  octahedral  faces  resembling 
cleavage.  Fracture,  uneven.  Brittle.  Color,  dark  gray 
to  iron-black;  opaque;  luster,  metallic,  fine  to  dull. 
Resembles  often  bits  of  iron  or  steel  in  the  rocks.  Streak, 
black.  Magnetic.  Hardness,  5.5-6.5.  Specific  gravity, 
5.2.  The  chemical  composition  is  Fe3O4  =  FeO  .  Fe2O3, 
or  FeO  =  31.0  per  cent,  Fe203  =  69.0.  Difficultly  fusi- 
ble before  the  blowpipe  and  in  the  oxydizing  flame  be- 
comes non-magnetic.  Slowly  soluble  in  hydrochloric  acid. 

Occurrence.  Magnetite  is  one  of  the  most  widely 
distributed  of  all  minerals.  It  is  found  in  f  all  kinds  of 
igneous  rocks,  usually  in  small  grains,  but  sometimes 
segregated  into  considerable  masses.  It  occurs  also  in 
rocks  produced  by  contact  metamorphism  and  in  the 
crystalline  schists,  sometimes  in  large  bodies.  It  is 
uncommon  in  the  unmetamorphosed  sedimentary  rocks. 
It  is  one  of  the  most  important  ores  of  iron. 

Determination.  The  appearance  of  magnetite  in  small 
dark  metallic-looking  particles  is  usually  sufficient  to  dis- 
tinguish it  in  the  rocks,  and  this  may  be  confirmed  by  a 


90  ROCKS  AND  ROCK  MINERALS 

test  of  its  hardness,  streak  and  magnetism,  together  with 
the  other  properties  described  above.  It  is  not  liable  to 
be  confused  with  any  other  mineral  except  ilmenite. 

SPINELS. 

Magnetite  may  be  regarded  as  the  type  of  a  group  of  minerals 
known  as  the  spinels.  They  have  the  general  chemical  composition 
RO  .  R2O3  and  crystallize  in  isometric  octahedrons  as  illustrated  in 
magnetite.  In  them  the  RO  is  either  MgO,  FeO,  MnO  or  ZnO  or 
mixtures  of  them;  R2O3  is  Fe2O3,  A12O3  or  Cr2O3  or  mixtures  of 
them.  True  spinel  is  MgAl2O4(MgO .  A12O3)  and  when  trans- 
parent and  of  good  color  is  sometimes  cut  as  a  gem.  Hercynite  is 
iron  spinel,  FeAl2O4,  and  chromite  is  FeCr2O4,  more  or  less  mixed 
with  other  spinel  molecules.  Depending  on  their  composition  the 
spinels  have  various  colors,  black,  green,  red  and  gray.  They  are 
extremely  hard,  7-8,  without  good  cleavage  and  are  of  high  luster 
to  pitchy.  Some  of  the  spinels  are  constituents  of  igneous  rocks, 
especially  of  those  low  in  silica  and  rich  in  iron  and  magnesia  like 
peridotite  and  dunite;  others  are  found  in  metamorphic  rocks, 
especially  those  produced  by  contact  metamorphism.  In  all  cases 
they  form  only  accessory  and  not  important  components  of  the 
rocks  and,  except  in  some  contact  rocks,  are  rarely  found  in  crystals 
sufficiently  large  to  make  them  of  megascopic  importance. 

ILMENITE. 

General  Properties.  Ilmenite  crystallizes  in  the  hex- 
agonal system  like  hematite,  but  it  is  so  rarely  seen  in 
good  megascopic  crystals  in  rocks  that  its  crystal  form 
is  not  a  matter  of  importance.  It  usually  occurs  in 
embedded  grains  and  masses,  sometimes  in  plates  of 
irregular  to  hexagonal  outline.  No  cleavage;  fracture, 
conchoidal;  brittle.  Color,  iron-black,  sometimes  with 
faint  reddish  to  brownish  tinge;  luster,  submetallic;  streak, 
black  to  brownish-red.  Opaque.  Hardness,  5-6.  Spe- 
cific gravity,  4.5-5.  Composition,  FeTiO3  =  FeO  .  TiO2. 
FeO  =  47.3,  Ti02  =  52.7.  Is  not  generally  pure,  but 
more  or  less  mixed  with  hematite,  Fe203,  with  which  it 
is  isomorphous.  Before  the  blowpipe  very  difficultly 
fusible;  in  the  reducing  flame  becomes  magnetic.  After 
fusion  with  carbonate  of  soda  can  be  dissolved  in  hydro- 


ROCK-MAKING  MINERALS  91 

chloric  acid  and  the  solution  boiled  with  tin  becomes 
violet  showing  titanium.  Fresh  mineral  difficultly  soluble 
in  acids;  decomposed  by  fusion  with  bisulphate  of  potash. 
The  solutions  give  reaction  for  iron  with  potassium 
ferricyanide.  The  test  for  titanium  is  the  safest  method 
to  determine  the  mineral. 

Occurrence.  Ilmenite  or  titanic  iron  ore,  as  it  is  often 
called,  is  a  widely  spread  mineral  occurring  as  a  common 
accessory  mineral  in  igneous  rocks  in  the  same  manner 
as  magnetite  which  it  often  accompanies.  In  the  same 
way  it  is  found  in  gneisses  and  schists.  Unless  the 
embedded  grains  are  of  such  size  that  they  can  be  safely 
tested  it  cannot  usually  be  discriminated  from  that 
mineral  by  simple  inspection.  The  most  important 
megascopic  occurrences  are  in  the  coarser  grained  gabbros 
and  anorthosites  where  the  mineral  is  very  common  and  is 
indeed  not  infrequently  segregated  in  places  into  such 
large  beds  and  masses  that  it  would  be  a  useful  ore  of  iron 
if  some  method  of  profitably  smelting  it  could  be  dis- 
covered. 

HEMATITE. 

Form.  Hematite  crystallizes  in  the  rhombohedral 
division  of  the  hexagonal  system  but  is  so  rarely  in  distinct 
well-formed  crystals  of  observable  size  as  a  rock  con- 
stituent that  this  is  not  a  matter  of  practical  importance. 

It  occurs  as  a  rock-mineral  in  three  different  forms:  as 
specular  iron  ore,  micaceous  hematite,  and  as  common  red 
hematite. 

In  the  first  case  it  forms  masses  and  plates,  the  latter 
sometimes  hexagonal  in  outline.  Its  color  is  black  to 
steel-gray  with  sometimes  a  faint  reddish  tone.  It  is 
opaque,  has  a  metallic  luster  which  is  sometimes  very  fine 
or  splendent  so  that  it  resembles  polished  steel  or  iron,  at 
other  times  it  is  rather  dull  but  metallic-looking.  Fracture, 
subconchoidal;  no  cleavage. 

As  micaceous  hematite  it  is  in  thin  flakes  which  some- 
what resemble  mica;  often  they  are  so  thin  as  to  be  trans- 


92  ROCKS  AND  ROCK  MINERALS 

lucent  and  then  have  a  deep  red  color.  The  luster  is 
submetallic  to  metallic,  sometimes  splendent  like  the 
specular  form.  The  thin  leaves  are  usually  of  ragged 
outlines  but  sometimes  hexagonal. 

Common  red  hematite  does  not  appear  crystallized. 
The  mineral  is  massive,  sometimes  columnar  or  granular, 
often  in  stalactitic  or  mamillary  forms  and  sometimes 
earthy.  It  is  dull,  without  metallic  luster,  opaque  and 
of  a  dark  red  color. 

General  Properties.  The  streak  of  hematite  is  of  a  red 
color,  from  bright  Indian  red  to  brownish  red  and  fur- 
nishes the  most  convenient  method  of  distinguishing  it 
from  magnetite  and  limonite.  Before  the  blowpipe  it  is 
very  difficultly  fusible  except  in  very  fine  splinters.  After 
heating  in  the  reducing  flame  magnetic.  Dissolves  slowly 
in  hydrochloric  acid  and  the  solution  gives  reactions  for 
iron. 

The  composition  is  Fe2Os,  ferric  oxide.  The  hardness 
varies  from  5.5-6.5;  specific  gravity  of  the  specular 
variety  is  5.2. 

Occurrence.  Hematite  is  one  of  the  most  widely  diffused 
of  minerals.  The  specular  variety  is  a  common  accessory 
component  of  feldspathic  igneous  rocks,  such  as  granite. 
It  is  also  found  in  the  crystalline  schists,  often  in  thick 
beds  and  masses. 

Micaceous  hematite  occurs  in  the  crystalline  schists 
in  megascopic  form,  as  in  itabirite,  and  also  in  minute 
microscopic  scales  it  is  the  red  coloring  matter  found  in 
igneous  and  metamorphic  rocks.  The  red  color  of  many 
potash  feldspars  is  due  to  it  and  so  is  that  of  many 
slates. 

Common  red  hematite  is  found  in  sedimentary  and 
metamorphic  rocks  in  beds  and  masses,  often  of  great 
size  and  forming  one  of  the  most  valuable  ores  of  iron. 
It  is  the  interstitial  cement  of  many  stratified  rocks,  such 
as  red  sandstones,  and  as  a  red  pigment  in  the  form  of 
powder  it  is  everywhere  distributed  in  all  classes  of  rocks 


ROCK-MAKING  MINERALS  93 

and  in  soils,  though  possibly  in  some  cases  it  may  be 
replaced  by  turgite  (hydrohematite),  2Fe203.H2O,  which 
often  closely  resembles  it.  Earthy  red  hematite,  usually 
more  or  less  mixed  with  clay,  is  called  red  ocher. 

LIMONITE. 

Form.  Limonite  does  not  crystallize,  but  occurs  in 
earthy  formless  masses  in  the  rocks,  and  when  found  in 
considerable  deposits  very  frequently  exhibits  compact 
stalactitic  or  mammillary  shapes  which  have  a  fibrous  or 
radiating  structure  and  are  sometimes  concretionary; 
sometimes  in  earthy  beds  or  deposits. 

General  Properties.  No  cleavage.  Luster  of  compact 
varieties  often  silky  to  sub-metallic,  but  generally  dull 
and  earthy.  Color,  various  shades  of  brown  from  very 
dark  to  brownish  yellow.  The  surface  of  the  compact 
stalactitic  or  mammillary  forms  often  has  a  varnish-like 
skin.  Opaque.  Streak,  yellow-brown.  The  hardness  of 
the  compact  mineral  varies  from  5-5.5  and  the  specific 
gravity  from  3.6-4.0.  The  composition  is  2  Fe203.3  H2O 
or  Fe2(OH)6.Fe203,  partly  dehydrated  ferric  hydroxide. 
Fe  =  59.8;  0  =  25.7;  H2O  =  14.5  =  100.  Difficultly 
fusible  before  the  blowpipe;  becomes  magnetic  in  the 
reducing  flame.  Heated  in  closed  glass  tube  gives  off 
water.  Slowly  soluble  in  hydrochloric  acid,  the  solution 
giving  reactions  for  iron.  The  yellow  streak  is  the  most 
convenient  means  of  distinction  from  hematite. 

Occurrence.  Limonite  occurs  in  several  different  ways. 
In  all  cases  it  is  strictly  a  secondary  substance  formed  at 
the  expense  of  previously  existing  minerals,  by  the  various 
agencies  of  weathering  and  alteration.  In  igneous  and 
metamorphic  rocks  it  is  frequently  seen  as  small,  earthy, 
yellowish  to  brownish  masses  which  represent  the  decay 
of  some  previous  iron-bearing  mineral,  such  as  pyrite, 
hornblende,  etc.  Accumulated  in  beds,  as  explained  under 
sedimentary  rocks,  it  frequently  has  the  compact  form 
with  stalactitic  and  mamillary  or  concretionary  structure. 


ROCKS  AND  ROCK  MINERALS 


As  bog  iron  ore  it  is  loose,  porous  and  earthy.  Mixed 
with  more  or  less  clay  it  forms  yellow  ocher  and  is  the 
yellow  pigment  of  many  soils  and  sedimentary  rocks. 

PYRITB. 

Form.  Pyrite  almost  invariably  occurs  in  crystals  in 
the  rocks,  very  seldom  in  grains  and  masses.  It  crystal- 
lizes in  the  isometric  system.  It  is  frequently  seen  in 
cubes  or  in  the  twelve-sided  form  seen  in  Fig.  47  and  called 
the  pyritohedron  because  this  mineral  so  commonly  shows 
it.  Combinations  of  the  two  are  also  very  common  as 


Fig.  47 


Fig.  48 


Fig. 49 


Fig.  50 


shown  in  Fig.  48.  Very  often  the  cubic  faces  are  striated 
by  fine  lines  as  seen  in  Fig.  50  produced  by  oscillating  or 
repeating  combinations  of  the  pyritohedron  on  the  cube 
faces.  The  octahedron  is  less  frequent  and  is  apt  to  be 
modified  by  the  pyritohedron  combining  with  it  as  in 
Fig.  49.  Other  more  complex  forms  also  occur. 

General  Properties.  No  good  cleavage;  fracture,  con- 
choidal  to  uneven.  Color,  brass  yellow;  luster,  metallic, 
splendent,  duller  when  tarnished.  Opaque.  Streak, 
greenish  to  brownish  black.  Hardness,  6-6.5;  specific 
gravity,  5.0.  Composition,  FeS2j  iron  =  46.6,  sulphur  = 
53.4  =  100.  Easily  fusible  before  the  blowpipe,  burning 
and  giving  off  sulphur  dioxide  gas,  and  leaving  a  magnetic 
globule.  In  the  closed  glass  tube  on  heating  gives  a  sub- 
limate of  sulphur  and  leaves  a  magnetic  residue.  Insol- 
uble in  hydrochloric  but  decomposes  in  boiling  nitric 
acid  with  separation  of  sulphur. 

The  color  and  crystallization  are  usually  sufficient  to 


ROCK-MAKING   MINERALS  95 

at  once  identify  pyrite  and  distinguish  from  other  rock 
minerals.  From  pyrrhotite,  FeuSi2,  and  chalcopyrite, 
FeCuS2,  other  sulphides  of  iron  which  occasionally  may 
be  seen  in  rocks,  the  test  of  hardness  discriminates  it 
from  chalcopyrite  (3.5)  which  can  be  readily  scratched 
with  the  knife  and  gives  reactions  for  copper;  pyrrhotite 
has  a  bronze  color,  is  also  scratched  by  the  knife  and  gives 
little  or  no  sulphur  in  the  closed  tube. 

Occurrence.  Pyrite  is  a  mineral  which  has  many 
different  modes  of  origin  and  in  consequence  is  found  in 
all  kinds  of  rocks  as  a  scattered  accessory  component, 
usually  in  small  distinct  crystals,  less  commonly  aggre- 
gated. The  largest  masses  are  found  in  ore  deposits, 
chiefly  formed  in  contact  zones  of  igneous  rocks  through 
the  action  of  mineralizing  solutions.  In  igneous  rocks  it 
appears  as  a  primary  product  of  crystallization  from  the 
molten  magma.  In  sedimentary  rocks  it  is  frequently 
found  replacing  fossils,  and  its  occurrence  must  be  due  to 
reactions  between  the  sulphur  of  albuminous  materials 
of  organic  life  and  the  iron  in  the  rocks.  It  is  common  in 

coal  seams. 

APATITE. 

Apatite  crystallizes  in  hexagonal  prisms  either  rounded  at  the 
ends  or  capped  by  a  six-sided  pyramid.  It  is  scratched  by  the 
knife,  has  a  vitreous  luster  and  is  white  to  green  or  brown  in  color. 
No  good  cleavage.  Brittle.  Transparent  in  small  crystals  to 
opaque  in  large.  Very  difficultly  fusible.  Dissolves  in  nitric  acid 
and  ammonium  molybdate  solution  added  to  a  few  drops  of  the 
nitric  acid  solution  gives  a  bright  yellow  precipitate  showing  the 
presence  of  phosphorus.  Composition  (CaF)Ca4(PO4)3;  phosphate 
of  lime  with  fluorine ;  the  fluorine  is  often  replaced  wholly  or  in  part 
by  chlorine.  Apatite  is  found  in  large,  sometimes  huge,  crystals  in 
pegmatite  dikes  and  in  metamorphosed  limestones  in  the  crystalline 
schists:  these  may  be  said  to  be  the  chief  megascopic  modes  of 
occurrence.  In  these,  however,  it  cannot  be  said  that  its  function 
as  a  rock-mineral  is  of  any  wide  or  general  importance.  In  addition 
to  this  it  occurs  in  minute  microscopic  crystals,  which  can  seldom 
be  detected  with  the  eye  or  lens,  in  all  kinds  of  igneous  rocks  and  in 
many  metamorphic  ones.  Microscopical  study  of  the  thin  sections 
of  such  rocks  has  shown  that  in  this  form  the  mineral  has  a  nearly 


96  ROCKS  AND  ROCK   MINERALS 

universal  distribution  as  a  constant  accessory  component.  Although 
the  relative  proportion  of  the  mineral  is  small,  rarely  rising  above 
two  or  three  per  cent  of  the  rock,  its  presence  is  a  matter  of  great 
importance,  since  by  it  the  phosphorus,  so  necessary  to  vegetable 
and  animal  life  (in  bones,  etc.),  is  furnished  to  the  soil  which  is 
formed  when  the  rocks  decay  and  break  down  under  the  action  of 
the  various  agents  of  weathering. 

SEC.  2.   Hydrous  Silicates. 

The  minerals  of  this  group  are  of  purely  secondary 
origin;  they  are  formed  from  previously  existent  ones 
by  the  agencies  of  weathering/  water  containing  carbon 
dioxide  or  vegetable  acids  and  by  heated  water  or  its 
vapors  circulating  in  already  solid,  existent  rocks.  Thus 
they  do  not  play  any  important  part  in  fresh  unchanged 
igneous  rocks;  only  as  these  alter  do  they  become  of 
importance  in  them;  their  true  home  is  in  the  meta- 
morphic  and  sedimentary  ones,  which  at  times  are  made 
up  wholly  of  these  minerals. 

The  important  ones  to  be  considered  in  this  section  are 
kaolin,  chlorite,  serpentine,  talc  and  zeolites.  Some  micas 
would  also  naturally  be  considered  here  and  among 
secondary  minerals  also  limonite,  but,  for  reasons  pre- 
viously stated,  these  have  been  treated  in  the  foregoing 
section. 

KAOLIN—  CLAY. 

Under  the  heading  of  clay  are  included  certain  hydrous 
silicates  of  alumina  having  well-known  physical  properties 
by  which  they  are  distinguished.  By  far  the  most  com- 
mon and  important  of  these  is  kaolin  which  may  be  taken 
as  a  type  of  the  group,  and  the  only  one  which  need  be 
considered  here  in  detail. 

General  Properties.  Kaolin  crystallizes  in  the  mono- 
clinic  system  forming  thin  plates  or  scales  often  with 
hexagonal  outlines  which  are  flexible  and  recall  mica  but 
are  inelastic;  these  are  generally  so  minute  and  aggre- 
gated together  that  the  crystal  form  is  not  a  matter  of 


ROCK-MAKING  MINERALS  97 

importance  in  megascopic  determination  of  the  substance. 
Usually  in  masses,  either  compact,  friable  or  mealy. 
Color  white,  often  tinted  yellow,  brown  or  gray.  Neither 
the  hardness  (2-2.5)  nor  the  specific  gravity  (2.6)  can  be 
used  for  practical  tests.  On  rubbing  between  the  fingers 
kaolin  has  a  smooth,  unctuous,  greasy  feel,  which  helps 
to  distinguish  it  from  fine  aggregates  of  some  other 
minerals  occurring  in  nature:  thus  its  presence  in  soils 
can  usually  be  told  by  rubbing  out  the  fine,  gritty 
particles  of  quartz,  feldspar,  etc.,  and  observing  if  there 
is  a  smooth,  unctuous  residue  of  clay. 

It  is  infusible  before  the  blowpipe,  but  moistened  with 
cobalt  nitrate  and  ignited  turns  blue  showing  presence  of 
alumina.  Heated  in  the  closed  glass  tube  it  yields  water. 
Insoluble  in  hydrochloric  acid.  In  the  phosphorus  bead 
before  the  blowpipe,  undissolved  silica  is  left;  this  test 
helps  to  distinguish  it  from  bauxite,  a  hydrated  oxide  of 
aluminum  (A12O(OH)4)  which  very  much  resembles  it 
and  sometimes  occurs  in  considerable  deposits.  Bauxite 
dissolves  in  the  phosphorus  bead  completely.  The 
chemical  composition  of  kaolin  is  H4Al2Si209'  a  combi- 
nation of  A1203  .  2  Si02  .  2  H20. 

Occurrence.  Kaolin  is  always  a  secondary  mineral 
formed  by  the  alteration  or  weathering  of  previously 
existent  aluminous  silicates  and  chiefly  feldspar.  The 
reaction  by  which  it  is  formed  from  alkalic  feldspar  is  one 
of  the  most  important  in  nature,  for  by  it  soil  is  chiefly 
made  and  the  alkali  necessary  for  plant  food  liberated 
and  converted  into  soluble  form.  It  is  expressed  as  fol- 
lows: 

Orthoclase  +  Water  +  Garb.  diox.  =      Kaolin      +  Potas.  Garb.  +  Quartz. 
2  KAlSi3O8  +  2  H2O  +        CO2        =  H4Al2Si2O9  +         KzCOs     +  4  SiO2. 


This  process  and  reaction  have  been  already  described 
under  feldspars.  The  feldspathoids  also  yield  kaolin  and 
the  process  could  be  expressed  as  follows  : 

Nephelite  +  Water  +  Garb.  diox.  =      Kaolin       +  Sodium  Garb. 
2  NaAlSiOi  +  2  HsjO  +        CO2        =    I^Al^Oo  + 


98  ROCKS  AND  ROCK  MINERALS 

They  are  more  apt  however  to  first  change  into  musco- 
vite  or  zeolites  and  these  ultimately  to  clay. 

From  what  has  been  said  it  is  clear  that  feldspathic 
rocks  furnish  kaolin,  and  every  stage  of  the  change  may  be 
observed  in  nature  as  described  more  completely  in  the 
chapter  dealing  with  the  origin  of  sedimentary  rocks  and 
soils.  Thus  kaolin  occurs  intimately  mixed  with  the 
feldspar  substance  of  such  rocks  as  are  undergoing  this 
change;  it  is  found  occasionally  in  quite  extensive  deposits 
where  such  rocks  have  been  completely  altered  in  place, 
and  the  products  of  decay  yet  remain  where  they  have 
been  formed,  and  lastly  it  occurs  in  extensive  beds  in  the 
sedimentary  formations.  Since  the  particles  of  kaolin 
are  very  minute,  light  and  flat  they  remain  much  longer 
in  suspension  than  the  other  products  of  land  waste,  and 
thus  in  erosive  and  sedimentary  processes  there  is  a  con- 
stant tendency  to  separate  them  from  the  other  particles. 
We  find  beds  of  clay  with  every  degree  of  admixture  with 
sand,  etc.,  that  pass  into  sandstones  and  other  rocks  but 
not  infrequently  they  are  of  a  high  degree  of  purity. 

CHLORITES. 

The  chlorites  are  an  ill-defined  group  of  hydrous  silicates 
so  named  on  account  of  their  green  color  (Greek  ^Xwpo's, 
green),  which  are  always  secondary  and  formed  at  the 
expense  of  previously  existing  silicate  minerals  which 
contain  aluminum,  iron  and  magnesia.  Outwardly  they 
resemble  the  micas,  but  unlike  them  their  folia  are  soft 
and  inelastic.  They  are  hydrous  silicates  of  aluminium 
with  ferrous  iron  and  magnesium.  They  have  certain 
common  properties  by  which  they  may  generally  be  easily 
recognized  as  a  group,  but  the  identification  of  the  different 
members  is  usually  a  difficult  matter  and  for  ordinary 
purposes  of  megascopic  petrography  of  little  importance. 
In  the  description  which  follows  then  it  is  these  general 
group  properties  which  are  given,  though  these  are  based 


ROCK-MAKING  MINERALS  99 

largely  on  the  species  clinochlore  which  is  perhaps  the 
most  common  and  best  known  of  the  group. 

Form.  The  chlorites  are  really  monoclinic  in  crystal- 
lization, but,  like  the  micas,  when  crystal  form  can  be 
observed  they  are  generally  in  six-sided  plates  and  tablets. 
More  commonly  they  occur  in  irregular  leaves  and  scales 
which  are  aggregated  together  into  fine  granular  or  coarse 
leafy  massive  forms  or  arranged  into  fan-like  or  rosette- 
like  groups.  The  scales  are  sometimes  flat;  often  bent  or 
curled. 

General  Properties.  Chlorite,  like  mica,  has  a  highly 
perfect  cleavage  in  one  direction  parallel  to  the  flat  base 
of  the  plates.  The  cleavage  leaves  are  flexible  and  tough 
but  unlike  mica  they  are  inelastic.  Luster  of  cleavage 
face  rather  pearly.  Color  green,  variable,  usually  a 
rather  dark  green.  Usually  translucent.  Hardness,  2-2.5 
soft,  just  scratched  by  the  finger  nail.  Specific  Gravity 
about  2.7.  Streak  pale  green  to  white.  The  chemical 
composition  of  the  chlorites  is  not  definitely  understood 
and  seems  to  be  complex:  it  may  be  illustrated  by  the 
formula  assigned  to  clinochlore,  H8(MgFe)5Al2Si30i8, 
which  may  be  written  4  H2O  .  5  (MgFe)O  .  A12O3  .  3  Si02: 
Ferrous  iron  and  magnesia  are  isomorphous.  In  kam- 
mererite,  a  rare  violet-red  variety,  part  of  the  alumina  is 
replaced  by  chromic  oxide,  Cr2O3.  Before  the  blowpipe 
chlorites  are  infusible  or  very  difficultly  so;  with  the 
borax  bead  they  react  for  iron.  Heated  in  the  closed 
glass  tube  they  yield  water.  They  are  insoluble  or 
difficultly  so  in  hydrochloric  acid  but  are  decomposed  in 
sulphuric  acid.  These  reactions  are  those  of  the  common 
kinds. 

Occurrence.  The  chlorites  are  a  widely  spread  group  of 
minerals,  and  occur  wherever  previously  existent  rocks 
containing  silicates  composed  of  alumina,  iron  and  mag- 
nesia, such  as  dark  micas,  amphibole,  pyroxene  and 
garnet,  are  being  altered  by  geologic  processes.  To 
chlorite  many  igneous  rocks  owe  their  green  color,  the 


100  ROCKS  AND  ROCK  MINERALS 

original  ferro-magnesian  silicates  having  been  broken 
down  by  decay  and  changed  more  or  less  completely  into 
this  substance.  They  are  apt  to  lose  their  original  bright, 
clean  appearance  and  hard  clear-cut  fracture  and  become 
dull  green  and  more  or  less  soft  and  even  earthy.  This 
change  can  also  be  often  observed  in  the  case  of  single 
embedded  crystals  of  the  above-mentioned  minerals, 
which  become  soft,  dull  green  masses. 

Chlorite  is  also  of  common  occurrence  in  the  schistose 
rocks;  in  chlorite-schist  it  is  the  prominent  component 
accompanied  by  other  minerals;  other  schists  often  owe 
their  green  color  to  its  presence,  as  in  green  slates  for 
example.  Thus  in  finely  disseminated  particles  it  is  a 
common  coloring  matter. 

SERPENTINE. 

General  Properties.  Serpentine  does  not  crystallize, 
and  therefore  has  no  crystal  form  of  its  own,  but  it  is 
sometimes  found  in  the  crystal  form  of  other  minerals 
which  have  been  altered  to  this  substance.  It  is  usually 
massive,  sometimes  finely  granular  or  even  slaty;  some- 
times fibrous,  the  fibers  fine,  flexible  and  easily  separable, 
like  asbestus.  Massive  varieties  have  a  conchoidal  or 
splintery  fracture.  Has  a  smooth,  greasy  feel.  The 
color  of  massive  varieties  is  green,  bright  yellowish  green, 
olive  green,  to  blackish  green,  or  nearly  black;  the  fibrous 
varieties  are  apt  to  be  brownish,  yellowish  brown,  pale 
brown  or  nearly  white.  Luster  of  the  massive  varieties 
greasy,  wax-like,  glimmering  and  usually  feeble  to  dull;  of 
fibrous  varieties  pearly  to  opalescent.  Translucent  to 
opaque.  Hardness,  2.5-3.0;  apparent  greater  hardness 
is  caused  by  presence  of  remains  of  the  original  mineral 
or  by  infiltrated  and  deposited  silica.  Specific  gravity 
somewhat  variable,  fibrous  2.2-2.4,  massive  2.5-2.7. 
Composition,  H4Mg3Si2O9  =  2  H2O  .  3  MgO  .  2  SiO2.  A 
small  part  of  the  MgO  is  usually  replaced  by  ferrous  oxide, 
FeO.  Before  the  blowpipe  difficultly  fusible,  fine  fibers 


ROCK-MAKING  MINERALS  101 

fuse  more  readily.  In  the  closed  glass  tube  yields  water 
on  ignition.  The  finely  powdered  or  divided  material 
decomposes  in  boiling  hydrochloric  acid  with  separation 
of  silica  but  does  not  gelatinize.  The  solution  may  be 
tested  for  iron  and  magnesia.  Easily  told  from  epidote 
and  other  common  green  silicates  which  may  resemble  it 
by  its  greasy  feel  and  softness. 

Occurrence.  Serpentine  is  a  secondary  mineral  result- 
ing from  the  alteration  of  previously  existing  silicates 
containing  magnesium.  Thus  pyroxene,  amphibole  and 
especially  olivine  may  be  altered  to  this  substance.  In 
the  case  of  olivine  the  process  can  be  illustrated  by  the 
following  equation : 

Olivine  +  Water   +  Carb.  diox.     =  Serpentine    +  Magnesite 
2  Mg2Si04  +  2  H2O   +      C02  =  H4Mg3Si209  +   MgCO3 

This  would  explain  the  frequent  association  of  the  mineral 
magnesite,  MgCOs,  with  serpentine;  or  it  might  be  taken 
into  solution  by  the  carbonated  water  and  removed. 

A  still  simpler  method  would  be  by  the  action  of  heated 
waters  containing  some  soluble  silica. 

3  Mg2Si04  +  4  H2O  +  SiO2  =  2  I^MggSiaOg. 

Therefore  as  a  product  of  alteration  of  such  minerals, 
especially  by  the  action  of  heated  waters,  serpentine  is  a 
common  and  widely  diffused  mineral  and  is  found  both 
in  igneous  and  metamorphic  rocks.  It  may  occur  dis- 
seminated in  small  scattered  masses  in  the  rocks  or  form 
large  independent  bodies  of  itself,  as  described  further 
under  the  chapters  dealing  with  the  rocks.  Besides  the 
common  massive  form,  many  sub-varieties  of  serpentine 
are  known;  the  most  important  of  these  is  the  finely 
fibrous  one,  often  taken  for  asbestus,  which  is  known  as 
chrysotile.  It  usually  occurs  in  seams  in  the  massive 
variety.  Bright  green  massive  material  is  known  as 
precious  serpentine  and  is  cut  for  ornamental  purposes. 


102  ROCKS  AND  ROCK   MINERALS 

TALC. 

General  Properties.  The  exact  crystal  form  of  talc  is 
doubtful,  but  this  is  not  a  matter  of  importance  since  it  so 
rarely  occurs  in  distinct  crystals.  It  is  usually  seen  in 
compact  or  strongly  foliated  masses,  sometimes  in  scaly 
or  platy  aggregates  which  may  be  grouped  into  globular 
or  rosette-like  forms.  Like  mica  it  has  a  perfect  cleavage 
in  one  direction,  but  the  laminae  though  flexible  are  not 
elastic;  it  is  sectile.  It  has  a  soft  greasy  feel.  The 
cleavage  face  has  a  mother  of  pearl  luster.  The  color  is 
white,  often  inclining  to  green;  apple-green;  sometimes 
gray  to  dark  gray.  Usually  translucent.  Hardness 
=  1-1.5,  easily  scratched  with  the  finger  nail.  Specific 
gravity,  2.7-2.8.  Streak,  light,  usually  easily  seen  on 
dark  cloth.  Composition,  H2Mg3(SiO3)4,  acid  metasili- 
cate  of  magnesium.  Before  the  blowpipe  it  whitens, 
exfoliates  and  fuses  with  difficulty  on  the  edges.  Only 
yields  water  in  the  closed  glass  tube  on  intense  ignition. 
Scarcely  acted  on  by  hydrochloric  acid.  It  is  easily 
recognized  by  the  properties  mentioned  above. 

.Occurrence.  Talc  is  a  secondary  mineral  produced  by 
the  action  of  circulating  fluids  on  magnesium  silicates, 
especially  those  free  from  alumina,  such  as  olivine,  hypers- 
thene  and  some  pyroxenes  and  amphiboles.  The  process 
could  be  illustrated  by  the  following  equation. 

Enstatite      +  Water  +  Garb.  diox.  =        Talc  +Magnesite 

4  MgSiO3      +    H2O  +        CO2        =  HsMg.,  (Si03)4  +   MgCO3 


Thus  talc  occurs  at  times  in  the  igneous  rocks  as  an 
alteration  product  of  such  silicates,  especially  in  the 
peridotite  and  pyroxenite  groups.  The  place,  however, 
where  it  plays  an  important  function  is  in  the  meta- 
morphic  rocks,  where  alone  it  may  form  independent 
masses,  as  in  steatite  or  soapstone,  or  be  an  important 
component  of  several  varieties  of  schistose  rocks  as  in 
talcose  schists. 


ROCK-MAKING  MINERALS  103 

ZEOLITES. 

The  zeolites  are  a  group  of  hydrous  silicates,  composed 
like  the  feldspars  of  aluminum  with  alkali  and  alkali- 
earth  metals.  They  are  indeed  for  the  most  part  second- 
ary minerals  which  have  been  formed  at  the  expense  of 
feldspars  and  feldspathoids  by  the  action  of  heated  cir- 
culating waters  and  steam  and  are  thus  chiefly  found  in 
igneous  and  especially  volcanic  rocks.  They  do  not 
form  a  group  so  closely  related  in  crystallization  and 
other  properties  as  the  feldspars,  but  still,  in  many  ways, 
they  have  certain  common  properties  by  which  they  may 
be  distinguished.  These  will  be  first  described,  and  then, 
out  of  the  many  species,  the  individual  characters  of  a 
few  of  the  most  important  will  be  treated. 

Group  Properties.  The  zeolites  are  nearly  always  well  crystallized, 
the  crystals  presenting  the  forms  characteristic  of  the  different 
species.  They  have  a  vitreous  luster,  are  usually  colorless  or  white, 
sometimes  tinted  yellow  or  red,  like  feldspar.  They  are  usually  of 
inferior  hardness  and  can  be  scratched  by  the  knife.  Their  specific 
gravity  is  low,  2.1-2.4.  They  fuse  very  readily  before  the  blowpipe, 
most  of  them  with  intumescence  (whence  the  name,  fe<V,  Greek, 
to  boil),  but  some  quietly,  to  white  glasses  or  enamels.  They  dis- 
solve in  hydrochloric  acid,  sometimes  gelatinizing  and  sometimes 
with  separation  of  slimy  silica.  Some  of  the  more  common  varieties 
are,  analcite,  natrolite,  stilbite  and  heulandite. 

Analcite.  This  zeolite  crystallizes  in  isometric  trapezohedrons 
like  garnet,  which  easily  enables  one  to  recognize  it.  Generally 
colorless  to  white.  Before  the  blowpipe  first  becomes  opaque,  then 
fuses  quietly  to  a  clear  glass,  coloring  the  flame  yellow.  Dissolves 
in  hydrochloric  acid  with  separation  of  silica  but  does  not  gelatinize. 
Its  composition  is  NaAl(SiO3)2  +  H2O. 

Natrolite.  Crystallizes  in  orthorhombic  prisms  which  are 
generally  long,  slender  and  even  needle-like  and  arranged  in  diver- 
gent bunches  or  compacted  into  fibrous,  often  radiating  masses. 
Before  the  blowpipe  fuses  easily  and  quietly  to  a  clear  glass;  fuses  in 
a  candle  flame.  Dissolves  in  acid  with  gelatinization.  Composition, 
Na2Al(AlO)  (SiO3)3  +  2  H2O. 

Stilbite.  Crystallizes  in  complex  monoclinic  crystals,  which  are 
usually  so  compounded  together  that  the  aggregate  has  the  form  of 
a  sheaf.  There  is  a  perfect  cleavage  in  one  direction  and  this  appears 


104  ROCKS  AND  ROCK  MINERALS 

on  the  side  of  the  sheaf  with  pearly  luster.  Sometimes  in  divergent, 
sometimes  in  globular  groups.  White  or  red  in  color.  Before  the 
blowpipe  swells,  intumesces  and  fuses  to  a  white  enamel.  Dissolves 
in  acid  without  gelatinization.  Composition, 

H4(CaNa2)Al2(SiO3)6  +  4  H2O. 

Heulandite.  Crystallizes  in  flattened  monoclinic  crystals  which 
aggregate  into  compound  individuals,  the  crystals  being  grown 
side  by  side  with  the  flattened  surfaces  together.  There  is  a  perfect 
cleavage  parallel  to  this  flattened  side  which  has  a  pearly  luster. 
The  cleavage  plates  are  often  curved  and  have  a  lozenge-shaped 
outline.  Blowpipe  and  chemical  characters  like  stilbite.  Com- 
position, H4CaAl2(SiO3)6  +.  3  H2O. 

Occurrence.  As  stated  above,  the  zeolites  are  second- 
ary minerals  chiefly  found  in  igneous  rocks.  They  are 
found  in  these  especially  when  they  have  been  subjected 
to  the  action  of  circulating  waters  and  steam  which  have 
attacked  the  feldspars  and  feldspathoids.  Thus,  for 
example,  a  mixture  of  albite  and  nephelite  with  water 
would  yield  analcite,  as  follows : 

Albite       +  Nephelite  +  Water  =          Analcite 
NaAlSi3O8  +  NaAlSiO4  +  2  H2O  =  2  NaAl(SiO3)2 .  H2O 

Thus  where  feldspathic  rocks  have  been  somewhat  altered 
they  are  very  apt  to  contain  zeolites  in  small  amounts 
scattered  through  them;  in  some  rare  cases  it  has  been 
found  that  a  considerable  part  of  the  rock  mass  is  com- 
posed of  them,  especially  of  analcite.  Ordinarily  the 
presence  of  these  minerals  is  not  to  be  detected  megascopi- 
cally,  though  it  may  be  discovered  by  heating  some  of  the 
powdered  rock  in  a  closed  glass  tube,  when  the  easy  evolu- 
tion of  abundant  water  would  indicate  their  presence. 

Their  especial  home,  from  the  megascopic  point  of  view, 
is  in  the  lavas,  particularly  basaltic  ones.  Here  they  are 
found  coating  and  lining  cavities  and  the  sides  of  jointing 
planes,  and  composing  the  materials  of  the  amygdaloids 
in  lavas,  as  described  under  amygdaloidal  structure  and 
under  basalt.  They  may  be  associated  with  crystals  of 


ROCK-MAKING  MINERALS  105 

quartz  and  of  calcite  in  such  occurrences,  and,  in  addition 
to  the  common  kinds  mentioned  above,  many  others  may 
occur  whose  description  must  be  sought  for  in  the  larger 
manuals  on  minerals. 

Carbonates. 

The  carbonates  are  salts  of  carbonic  acid,  H2C03,  and 
are  secondary  minerals,  in  that  their  metals  have  been 
derived  from  previously  existent  minerals  acted  upon  by 
water  and  carbon  dioxide  either  from  the  supply  already 
in  the  atmosphere  or  coming  from  interior  sources  deep 
within  the  earth.  The  carbonates  thus  formed  have  been 
either  deposited  directly  where  we  now  find  them,  or,  being 
soluble  in  water  containing  carbon  dioxide,  they  have  been 
carried  in  solution  into  lakes  and  into  the  sea  and  rede- 
posited  by  the  agencies  of  organic  life  in  the  form  of 
chalk,  limestone,  etc.  The  shifting  about  of  carbonates 
on  the  earth's  surface,  owing  to  their  solubility  in  water 
containing  carbon  dioxide,  is  a  geologic  process  of  great 
importance  and  gives  rise  to  a  variety  of  products  which 
are  described  in  their  appropriate  places  as  rock  forma- 
tions. Here  they  are  treated  simply  as  minerals  and  out 
of  the  considerable  number  of  kinds  only  two  are  of  such 
importance  that  they  will  be  considered  in  detail  —  calcite 
and  dolomite. 

CALCITE. 

Form.  Calcite  crystallizes  in  the  rhombohedral  system. 
The  crystals  are  often  very  fine,  perfect,  and  sometimes 
of  very  large  size.  It  displays  a  great  variety  of  crystal 
forms  many  of  them  being  often  very  complex.  Some 
simple  ones  are  shown  in  the  annexed  figures.  Fig.  51  is 
a  simple  flat  rhombohedron,  three  faces  above  and  three 
below.  Fig.  52  is  the  unit  rhombohedron,  so  called 
because  the  faces  are  parallel  to  the  cleavage.  Fig.  53  is 
a  very  acute  rhombohedron.  Fig.  54  is  a  very  short 
prism  with  six  prism  faces  ra  and  the  flat  rhombohedron 
e  above  and  below;  Fig.  55  is  similar  but  the  prism  faces 


106 


ROCKS  AND  ROCK  MINERALS 


w  are  elongated.     Fig.  56  is  an  acutely  pointed  form,  the 
scalenohedron.     All  of  these  are  common  crystal  forms 


Fig.  5i 


Fig. 


Fig.  53 


Fig.  54 


shown  by  calcite;   they  occur  when  it  is  found  lining 
cavities  in  rocks,  in  druses,  in  amygdaloids,  in  geodes  and 
on  the  surfaces  of  joint  planes  and  fissures  and  in  caves; 
in  short  in  all  those  places  where  it  could  be  deposited  by 
infiltrating  water  containing  it  in  solution.     As  a  rock- 
making  mineral  it  is  massive;  of- 
ten crystalline-granular  and  coarse 
to  fine  in  structure  as  in  marble,  or 
compact  as  in  ordinary  limestone, 
or  loose  and  powdery  in  texture  as 
in  chalk.     It  is  sometimes  spongy 
in  structure  as  in  tufa,  or  rounded, 
stalactic,    mamillary,    etc.,   as    in 
cave  deposits  and  in  concretions, 
and  not  uncommonly  fibrous. 
Cleavage.     Calcite  has   a   very   perfect   rhombohedral 
cleavage;  in  three  directions  parallel  to  the  faces  or  of  the 
crystal  shown  in  Fig.  52.    While  this 
is,  of  course,  best  produced  in  iso- 
lated crystals  it  can  be  well  observed 
on  the  fractured  surfaces  of  coarsely 
crystalline  massive  forms,  as  in  many 
marbles  and  related  rocks  and  in  the 
***•  57-  massive  calcite  of  veins.    The  angles 

of  the  face  of  the  rhomb  produced  by  cleavage  are  just 
about  78  and  102  degrees,  as  shown  in  Fig.  57,  and  small 


Fig-  55 


102° 


Fig.  56 


ROCK-MAKING   MINERALS  107 

cleavage  pieces  can  be  readily  tested  by  applying  them 
to  the  edges  of  the  figure  on  the  paper. 

General  Properties.  The  hardness  is  3;  readily  scratched 
or  cut  by  the  knife;  chalky  varieties  are  of  course  softer. 
The  specific  gravity  is  2.71  if  pure.  The  natural  color 
of  calcite  is  colorless  or  white  but  it  frequently  displays  a 
great  variety  of  exotic  colors  owing  to  the  presence  of 
impurities;  thus  it  may  be  reddish  or  yellowish  from  iron 
oxides  or  gray  to  black  from  organic  matter,  or  green, 
purple,  blue,  etc.,  from  ether  substances.  Streak,  white 
to  gray.  The  luster  of  the  crystallized  calcite  is  vitreous; 
of  massive  forms  glimmering  to  dull.  In  the  same  way 
it  varies  from  transparent  to  translucent  to  opaque. 
Chemical  composition,  CaC03;  or  CaO  .  C02.  CaO  =  56 
per  cent,  C02,  44  per  cent  =  100. 

Before  the  blowpipe  it  is  infusible,  colors  the  flame 
reddish  yellow  and  after  ignition  if  placed  on  moistened 
yellow  turmeric  paper  colors  it  brown.  Fragments 
effervesce  freely  in  cold  (difference  from  dolomite)  very 
dilute  acids. 

Occurrence.  Calcite  is  one  of  the  most  common  and 
widely  diffused  of  all  minerals.  It  is  found  in  the  igneous 
rocks  as  the  result  of  alteration  of  the  lime-bearing  sili- 
cates by  waters  containing  carbon  dioxide  in  solution, 
other  substances  being  formed  as  by  products  at  the  same 
time.  Thus,  for  example, 

Pyroxene-     +  Water   4-  Garb.  diox.  =    Calcite     +   Serpentine  +  Quartz 
3  CaMg(SiO3)2  +   2  H2O   +      3  CO2       =   3  CaCO3  +  H4Mg3Si2O9  +  4  SiO2 


The  calcite  thus  formed  may  remain  for  a  time  in  the 
rock  but  eventually,  as  the  latter  breaks  down  into  soil, 
it  is  to  a  greater  or  lesser  extent  removed  in  solution  and 
carried  away.  The  mineral  has  also  been  found  to  occur 
very  commonly  in  minute  cavities  in  unaltered  igneous 
rocks,  especially  intrusive  ones,  and  its  origin  is  probably 
due  to  infiltration  and  deposition  of  the  material  from 
neighboring  rock  masses.  In  these  cases  just  mentioned 


108  ROCKS  AND  ROCK  MINERALS 

the  mineral  is  ordinarily  not  observable  megascopically, 
but  its  presence  is  easily  ascertained  by  immersing  a 
fragment  of  the  rock  in  cold  dilute  acid  and  seeing  if  it 
effervesces  and  gives  off  carbonic  acid  gas.  Calcite  also 
occurs  in  amygdaloidal  cavities  in  lavas,  especially  in 
basalts,  and  often  in  good  crystals. 

In  the  sedimentary  and  metamorphic  rocks  calcite 
plays  a  much  more  important  part.  It  is  very  commonly 
found  distributed  through  them  in  fine  particles  or  acting 
as  a  cement  to  the  other  mineral  granules.  From  this 
role,  if  we  examine  a  whole  series  of  these  rocks,  we  find 
it  increasing  more  and  more  in  abundance  and  importance 
as  a  constituent,  until  finally  there  are  enormous,  widely 
extended  rock-masses,  such  as  the  chalks,  limestones 
and  marbles,  composed  practically  and  in  some  instances 
wholly  of  this  substance.  Such  rocks  are  found  described 
in  their  appropriate  places  in  this  work ;  it  is  sufficient  here 
to  mention  that  in  the  sedimentary  rocks  calcite  plays  an 
important  part  in  chalk,  limestone,  calcareous  marls, 
calcareous  sandstones,  etc.;  in  chemical  deposits  in 
calcareous  tufas,  sinters,  stalagmitic  deposits,  veins,  etc., 
and  in  the  metamorphic  rocks  in  marbles  and  in  rocks 
which  are  mixtures  of  calcite  and  various  silicates. 

Determination.  Calcite  when  sufficiently  coarsely  crys- 
tallized is  easily  recognized  by  its  inferior  hardness  and 
rhombohedral  cleavage.  This  is  confirmed  chemically 
by  its  ready  solubility  in  cold  dilute  acids  with  efferves- 
cence of  062  gas  and  if  necessary  a  test  for  the  presence 
of  lime.  For  the  distinction  from  dolomite,  reference 
should  be  made  to  the  description  of  that  mineral. 

DOLOMITE. 

Form.  Dolomite  crystallizes  in  the  rhombohedral 
system  and,  like  calcite,  it  is  found  in  simple  rhombo- 
hedral crystals  whose  faces  are. parallel  to  the  cleavage; 
Fig.  52  of  calcite.  Unlike  that  mineral  it  rarely  occurs 
in  complicated  crystals  and  the  simple  rhombohedron,  in 


ROCK-MAKING  MINERALS  109 

which  it  is  generally  seen  when  showing  outward  crystal 
form,  usually  has  its  faces  curved  as  represented  in  Fig. 
58  instead  of  flat.  Moreover  the  curved  crys- 
tals are  apt  to  be  compound,  made  up  of  a 
number  of  sub-individuals.  This  is  the  way  it 
occurs  when  lining  druses  and  cavities,  but  as 
a  rock-making  mineral  it  is  nearly  always 
massive,  often  crystalline-granular  and  coarse 
to  fine  in  texture  as  in  some  marbles.  It  is  also  often 
compact- massive  as  in  some  limestones;  more  rarely 
columnar  or  fibrous. 

Cleavage.  The  cleavage,  like  that  of  calcite,  is  perfect 
in  three  directions  parallel  to  the  faces  of  the  simple 
rhombohedron.  The  angles  of  the  cleavage  rhombs 
differ  only  a  degree  or  so  (74  and  106  degrees,  nearly) 
from  those  of  calcite  and  therefore  by  form  alone  they 
cannot  be  distinguished  by  the  eye. 

General  Properties.  The  natural  color  is  white  and 
while  this  is  often  seen  the  mineral  is  very  apt  to  be 
tinted  some  exotic  color  by  other  substances;  thus  it  may 
be  reddish,  brown,  greenish,  gray  or  even  black.  Luster, 
vitreous,  sometimes  pearly  to  dull  or  glimmering  in  com- 
pact varieties.  Translucent  to  opaque.  The  hardness 
is  3.5-4.0,  harder  than  calcite  but  easily  scratched  with  a 
knife.  Specific  gravity,  2.8-2.9.  Chemical  composition, 
CaMg(C03)2;  CaO  30.4,  MgO  21.7.  C02  47.9  =  100. 
Before  the  blowpipe  infusible,  but  placed  on  moist  tur- 
meric paper  after  ignition  colors  it  brown.  Does  not 
dissolve  or  is  very  little  acted  upon  in  cold  dilute  acid  but 
on  boiling  effervesces  and  goes  into  solution  (difference 
from  calcite).  The  solution  may  be  tested  for  lime  and 
magnesia  as  directed  in  the  chapter  on  mineral  tests. 

Occurrence.  Dolomite  occurs  as  a  scattered  accessory 
component  of  certain  crystalline  schists  and  in  beds  of 
gypsum,  etc.,  but  its  chief  importance  as  a  rock-making 
mineral  lies  in  the  fact  that  alone  it  forms  immense 
extended  beds  both  in  the  sedimentary  series  and  in  the 


110  ROCKS  AND  ROCK  MINERALS 

metamorphic  rocks.  It  thus  exactly  parallels  calcite, 
and  in  limestones  and  in  marbles  we  have  every  degree  of 
transition  between  these  two  substances,  thus  marbles 
composed  of  calcite  alone,  others  with  increasing  amounts 
of  dolomite  until  pure  dolomite  marble  is  reached.  This 
is  more  fully  described  under  the  carbonate  rocks. 

Determination.  The  rhombohedral  cleavage  and  infe- 
rior hardness  separate  dolomite,  like  calcite,  from  other 
common  rock  minerals.  The  frequent  curved  surfaces 
help  to  distinguish  it  from  calcite  but  the  test  with  hot 
and  cold  acid  mentioned  above,  together  with  the  finding 
of  magnesia  in  the  solution,  is  the  only  safe  way. 

Siderite,  Magnesite  and  Breunerite.  As  an  appendix  to  calcite 
and  dolomite  these  carbonates,  which  are  sometimes  of  local  impor- 
tance, may  be  mentioned.  Siderite  or  spathic  iron  ore  is  FeCOg, 
ferrous  carbonate,  while  magnesite  is  magnesium  carbonate,  MgCOa, 
and  breunerite  is  an  isomorphous  mixture  of  the  two,  (MgFe)CO3. 
In  crystallization,  cleavage,  hardness,  etc.,  they  closely  resemble 
the  other  carbonates  described.  Siderite  is  usually  light  to  dark 
brown  in  color;  magnesite  white;  breunerite  brownish.  Siderite 
chiefly  occurs  more  or  less  massive  and  impure  in  certain  sedimentary 
deposits,  in  the  so-called  "  clay  iron  stone  "  and  is  a  valuable  ore  of 
iron.  Magnesite  occurs  chiefly  in  certain  metamorphic  rocks  and 
is  apt  to  be  associated  with  serpentine,  talc,  etc.  It  may  be  acccom- 
panied  or  replaced  by  breunerite. 

Sulphates. 

The  sulphates,  like  the  carbonates,  are  in  general 
minerals  of  a  secondary  nature;  the  metals  they  contain 
have  been  taken  from  previously  existent  minerals,  the 
sulphuric  acid  has  been  furnished  for  the  most  part  by 
the  oxidation  of  metallic  sulphides  or  by  exhalations  in 
regions  of  igneous  activity.  With  a  few  exceptions  they 
are  readily  soluble  and  the  great  bulk  of  them,  which  has 
been  formed  during  geologic  time,  has  therefore  been 
transferred  to  the  sea,  which,  with  the  salt  lakes  in  the 
interior  of  continents,  is  now  the  great  reservoir  of  these 
substances  as  well  as  of  many  other  soluble  salts  such  as 


ROCK-MAKING  MINERALS 


111 


the  chlorides.  As  rock-making  minerals  only  two  of  the 
large  number  of  sulphates  known  are  of  importance, 
gypsum  and  anhydrite.  Barite,  BaSO-i,  which  is  one  of 
the  few  insoluble  sulphates,  is  a  very  common  material  in 
veins  and  is  also  found  in  concretions,  but  it  does  not 
form  independent  rock-masses  or  play  any  role  as  a  rock- 
component  as  the  two  first  mentioned  do. 

GYPSUM  —  SELENITE. 

Form.  Gypsum  crystallizes  in  the  monoclinic  system, 
and  the  common  form  of  the  crystals  is  shown  in  Fig.  59. 
The  same  crystal  is  shown  in  Fig.  60  revolved  so  that  the 
side  face  b  is  parallel  with  the  plane  of  the  paper;  such 
crystals  may  be  roughly  tested  by  placing  them  on  the 


Fig. 60 


Fig.  61 


diagram  and  seeing  if  the  angles  coincide.  Twin  crystals 
are  common  and  they  are  apt  to  assume  arrow-head 
forms  as  shown  in  Fig.  61.  More  commonly  as  a  rock 
constituent,  gypsum  occurs  massive,  foliated  often  with 
curved  surfaces,  or  granular  to  compact  and  sometimes 
fibrous. 

Cleavage.  Gypsum  has  a  perfect  cleavage  parallel  to 
the  side  face  6;  by  means  of  it  on  good  material  very  thin 
sheets  with  perfect  luster  may  be  split  off,  almost  as  in 
mica.  Such  sheets  will  be  found  to  break  in  one  direction 


112 


ROCKS  AND  ROCK  MINERALS 


Fig.  62 


in  straight  lines  with  a  conchoidal  fracture;  this  is  due  to 
another  cleavage  parallel  to  the  vertical  edge  between 
mm.  If  such  sheets  be  bent  cracks  will 
appear  in  them  making  angles  of  66  and 
114  degrees  with  the  straight  fracture 
edge  mentioned  above;  if  the  bending 
parallel  to  this  direction  is  continued  the 
plate  will  break  with  a  fibrous  fracture, 
and  a  cleavage  rhomb  like  that  shown  in 
Fig.  62  may  be  obtained.  In  massive 
coarsely  crystalline  gypsum,  these  cleav- 
ages can  usually  be  readily  obtained  and 
furnish  one  means  of  helping  to  identify 
it;  in  fibrous  material  it  simply  cleaves  parallel  to  the 
fibers;  in  the  compact  massive  forms  it  may  happen  that 
no  cleavage  is  seen. 

General  Properties.  The  natural  color  of  gypsum  is 
colorless  or  white,  and  crystals  are  transparent  to  trans- 
lucent, but  it  is  frequently  tinted  reddish  or  yellowish  or 
in  massive  varieties  may  be  even  red,  brown  or  black 
through  impurities,  and  translucent  to  opaque.  The 
luster  of  the  cleavage  face  b  is  glassy  to  pearly,  of  fibrous 
varieties  satiny,  while  massive  forms  are  glistening, 
glimmering  to  dull.  Streak,  white.  Hardness,  1.5-2.0, 
easily  scratched  by  the  finger  nail.  Specific  gravity  of 
pure  crystals,  2.32.  Chemical  composition,  hydrous  sul- 
phate of  calcium,  CaSO4  +  2  H2O.  CaO  32.5,  S03  46.6, 
H2O  20.9  =  100.  Before  the  blowpipe  fuses  easily  and 
after  ignition  colors  moistened  turmeric  paper  brown. 
Fused  with  carbonate  of  soda  and  charcoal  dust  on  char- 
coal and  transferred  to  a  moistened  surface  of  silver 
stains  it  dark.  Finely  powdered  mineral  is  readily 
soluble  in  boiling  dilute  hydrochloric  acid.  Heated 
intensely  in  a  closed  glass  tube  gives  off  water  and  becomes 
opaque.  Heated  moderately  (not  above  200  degrees) 
it  loses  some  water  and  becomes  plaster  of  Paris  and  the 
powder,  if  moistened,  again  takes  up  water  and  sets  or 


ROCK-MAKING  MINERALS  113 

becomes  solid  turning  back  into  gypsum.  If  heated  too 
highly  it  loses  all  its  water,  becomes  anhydrite,  CaSC>4, 
and  is  then  called  dead  burnt  plaster  and  does  not  set  as 
described  above. 

The  occurrence  of  gypsum  is  mentioned  later  in  its 
description  as  a  stratified  rock. 

ANHYDRITE. 

General  Properties.  Anhydrite  crystallizes  in  the  ortho- 
rhombic  system  but  in  the  rocks  in  which  it  occurs  it  is 
seen  in  granular  to  compact  masses,  less  commonly  in 
fibrous  or  foliated  forms.  It  has  a  cleavage  in  three 
directions  at  right  angles,  and  if  coarsely  crystalline  this 
may  be  observed  to  produce  cube-like  forms.  Usually 
white  but  sometimes  tinted  as  in  gypsum;  luster  of 
cleavage  faces  pearly  to  glassy;  in  massive  varieties  varies 
to  dull.  Harder  than  gypsum  =  3-3.5  but  easily  cut  by 
knife.  Specific  gravity,  2.95.  Chemical  composition, 
CaSO4;  CaO  41.2,  S03  58.8  =  100.  Blowpipe  and  other 
reactions  as  with  gypsum,  but  it  does  not  yield  water  in 
the  closed  glass  tube  on  heating  which  is  the  best  dis- 
tinction from  gypsum;  the  difference  in  cleavage  also 
aids  in  the  discrimination. 

Occurrence.  Like  gypsum,  anhydrite  forms  inter- 
stratified  beds  in  sedimentary  formations  especially  in 
limestones  and  shales.  It  is  also  found  in  masses  and  in 
geodes  in  such  rocks  and  is  a  common  associate  of  rock 
salt  and  gypsum. 

ROCK  SALT  —  HALITE. 

Rock  salt,  sodium  chloride,  NaCl,  is  the  only  chloride  which 
occurs  as  a  rock-forming  constituent  in  such  amounts  as  to  be  of 
importance.  It  is  easily  recognized  by  its  cubic  crystals,  perfect 
cubic  cleavage,  solubility  and  saline  taste.  Colorless  and  trans- 
parent to  white,  translucent;  frequently  tinted  various  colors  by 
impurities.  Hardness  =  2.5.  Occurs  in  beds,  sometimes  of  enor- 
mous thickness,  in  the  sedimentary  formations,  usually  clays  or 
shales,  and  is  frequently  accompanied  by  gypsum  and  anhydrite. 


CHAPTER  V. 
THE  DETERMINATION  OF  ROCK  MINERALS. 

THE  more  important  of  the  physical  properties  of  the 
rock  minerals  have  been  described  in  the  previous  chap- 
ter, and  in  most  cases  the  methods  by  which  these  are 
to  be  determined  have  been  stated  under  them.  In  the 
present  chapter  it  is  proposed  to  present  a  number  of 
qualitative  chemical  tests,  which  can  generally  be  made 
with  a  few  reagents  and  simple  apparatus,  and  which  are 
of  great  service  in  mineral  determination  and  in  thus 
aiding  to  classify  rocks.  This  is  followed  by  a  set  of 
tables;  one  for  rough  approximations  in  the  field,  the  other 
for  more  complete  identifications  in  the  laboratory  by 
means  of  the  properties  and  methods  described. 

Chemical  Tests. 

These  consist  mainly  in  observing  the  effect  of  acids 
upon  the  mineral,  whether  it  is  dissolved  or  only  partly 
attacked  or  is  wholly  insoluble;  if  soluble,  or  partly  so,  in 
ascertaining  with  certain  reagents  what  substances  have 
gone  into  solution.  These  serve  to  detect  the  acid  radicals 
and  metallic  oxides  which  alone  or  in  combination  com- 
pose the  rock  minerals.  A  few  useful  additional  tests  are 
given. 

A.  Powdering  the  Sample.  The  first  thing  in  testing 
the  solubility  of  minerals  is  to  prepare  a  finely  powdered 
sample.  Small  chips,  grains  or  splinters  of  the  sub- 
stance about  the  size  of  wheat  grains  and  as  pure  as 
possible  are  successively  crushed  and  ground  to  a  fine 
powder,  like  flour,  in  a  diamond,  steel  or  agate  mortar, 
until  a  sufficient  amount  has  been  produced.  It  is 
usually  best  to  crush  the  fragments  in  the  steel  mortar, 

114 


THE  DETERMINATION  OF  ROCK  MINERALS      115 

and  unless  iron  is  to  be  tested  for  they  may  be  ground  in 
this  as  well.  The  finer  the  powder  is  ground  the  more 
readily  it  will  go  into  solution;  it  is  therefore  generally 
best  to  grind  it  until  it  no  longer  feels  gritty  when  a  small 
pinch  is  rubbed  between  the  fingers. 

B.  Treatment  with  Acid.     A  small  bulk  of  the  powder 
prepared  as  above,  about  equal  to  a  pea  in  volume,  is  put 
in  a  test  tube,  covered  with  about  an  inch  of  distilled 
water  and  a  few  drops  of  nitric  or  hydrochloric   acid 
added.     Either  may  be  used,  but  if  it  is  desired  to  test 
for  phosphorus  or  chlorine  subsequently  in  the  solution 
the  former  should  be  employed.     The  test  tube,  if  the  cold 
acid  has  no  apparent  effect  upon  the  substance,  may  then 
be  gently  heatea  over  the  flame  of  a  bunsen  burner  or 
lamp  of  a  suitable  kind  until  the  liquid  is  brought  to 
boiling.     If  the  effect  is  slight  or  apparently  none  has 
been  produced  more  acid  is  added  and  the  boiling  repeated 
until  the  substance  is  brought  into  solution  or  it  is  appa- 
rent that  it  cannot  be  dissolved. 

C.  Carbonic    Acids  —  Carbonates.     If    the    substance 
effervesces  freely  and  readily  in  cold  dilute  acid  it  indicates 
that  it  is  a  carbonate  and  that  C02  gas  is  being  given  off. 
It  is  probably  calcite.     The  carbonates  of  magnesia  and 
iron  (dolomite,  siderite,  etc.)  are  scarcely  acted  upon  or 
but  very  slowly  in  cold  acid  but  effervesce  freely  when  the 
liquid  is  heated.     This  serves  as  a  convenient  means  of 
distinction  between  calcite  and  dolomite  which  may  be 
confirmed  by  tests  for  lime  and  magnesia  as  given  beyond. 
The  rare  rock  mineral  cancrinite,   a  silicate  containing 
CO2  also  effervesces  very  slowly  in  hot  dilute  acid;  the 
heated  solution  should  be  examined  in  a  good  light  with 
a  lens  when  a  slow  persistent  evolution  of  minute  bubbles 
will  be  seen.     In  regarding  the  heated  solution  care  must 
be  taken  not  to  confuse  the  ebullition  of  steam  bubbles 
with  the  effervescence  of  C02  gas;  a  moment's  pause 
should  be  given  to  allow  the  former  to  cease. 

D.  Soluble    Silicates.      Gelatinization.      If    the    sub- 


116  ROCKS  AND  ROCK  MINERALS 

stance  treated  according  to  A  wholly  or  partly  dissolves 
without  effervescence  it  should  be  tested  for  soluble 
silica.  If  only  partly  attacked  the  insoluble  portion 
should  be  filtered  off  and  the  filtrate  used.  This  is  con- 
centrated by  boiling  it  down  in  the  test  tube,  the  latter 
being  continuously  and  gently  shaken  to  prevent  crack- 
ing, until  the  solution  is  greatly  concentrated,  if  necessary 
to  a  few  drops.  It  is  then  allowed  to  cool  and  stand,  and 
if  it  becomes  a  jelly  the  presence  of  soluble  silica  is  indi- 
cated. If  the  amount  of  silicate  which  has  gone  into 
solution  is  relatively  large,  a  jelly  will  probably  form 
while  the  solution  is  being  boiled  down  and  is  still  hot, 
otherwise  the  solution  must  be  concentrated  and  allowed 
to  stand,  as  just  stated;  in  the  latter  case  care  must  be 
taken  not  to  confuse  a  thickening  of  the  solution  from 
concentration  in  it  of  the  salts,  especially  basic  salts  of 
iron,  with  the  true  jelly  of  soluble  silica.  If  the  solution 
is  carefully  carried  to  dryness  and  the  residue  heated  for  a 
few  moments  the  salts  on  being  moistened  with  strong 
hydrochloric  acid  and  then  warmed  will  go  into  solution 
in  water  while  the  silica  is  left  as  an  insoluble  residue  and 
may  be  filtered  off.  In  the  filtrate  the  various  metallic 
bases,  aluminum,  iron,  calcium,  etc.,  which  may  be  in 
solution,  can  be  tested  for  by  the  methods  described 
beyond. 

The  rock-making  silicates  which  will  go  into  solution  on 
boiling  with  nitric  or  hydrochloric  acid  are  nephelite, 
sodalite,  analcite,  olivine,  chondrodite,  serpentine,  anor- 
thite,  leucite,  noselite,  stilbite,  heulandite  and  cancrinite. 
All  except  serpentine,  leucite,  analcite,  stilbite  and 
heulandite  yield  gelatinous  silica;  with  these  when  the 
liquid  is  boiled  it  turns  from  the  milkiness  caused  by  the 
suspended  material  to  a  translucent  appearance  with 
slimy  silica  suspended  in  it.  The  bases,  however,  have 
gone  into  solution. 

E.  Insoluble  Silicates.  Fusion.  Most  rock-making  sil- 
icates are  insoluble  in  acids  or  only  partially  soluble.  To 


THE  DETERMINATION  OF  ROCK  MINERALS      117 

get  them  into  solution  so  that  the  bases  may  be  tested  for 
as  in  the  following  sections,  a  preliminary  process  of 
fusion  with  sodium  carbonate,  Na2COs,  must  be  under- 
taken. For  this  purpose  some  of  the  powder  obtained 
in  A  is  mixed  with  about  4-5  times  its  weight  of  dry 
anhydrous  sodium  carbonate,  placed  in  a  platinum 
crucible  or  spoon,  and  gently  heated  to  redness  over  a 
Bunsen  lamp  flame.  If  no  crucible  is  at  hand  a  coil  of 
platinum  wire  can  be  used  instead;  the  mixed  powder  is 
made  into  a  thick  paste  with  a  little  water  and  a  quantity 
taken  on  the  coil  and  carefully  fused  before  the  blowpipe. 
A  fused  bead  or  mass  the  size  of  a  large  pea  may  be 
obtained  in  this  way.  The  fusion  must  be  conducted 
until  bubbling  has  ceased. 

In  this  fusion  the  silicates  are  decomposed,  silica  is  liberated 
from  them  and  takes  the  place  of  the  carbonic  acid  in  the  sodium 
carbonate  which  is  thus  converted  into  sodium  silicate.  The  liberated 
CO2  gas  is  given  off  with  bubbling  and  frothing  of  the  fusion  and 
this  effect  is  in  itself  indicative  of  the  presence  of  silica  in  the  original 
substance,  provided  it  is  known  not  to  come  from  combined  water 
by  previous  trial.  The  reaction  might  be  illustrated  in  the  case  of 
pyroxene  as  follows: 

MgCaSi2O6  +  2  Na2CO3  =  2  Na2SiO3  +  MgO  +  CaO  +  2  CO2 

The  fused  mass  obtained  is  broken  up  in  a  diamond 
mortar,  placed  in  a  test  tube,  and  then  treated  with  acid 
until  dissolved,  evaporated  and  the  silica  separated  and 
the  metallic  bases  brought  into  solution  just  as  directed 
for  soluble  silicates  in  D. 

If  the  fusion  has  been  made  in  a  platinum  crucible  the 
cake  can  generally  be  loosened  and  removed  by  boiling  it 
with  a  little  water;  if  not,  it  is  dissolved  with  water  and  acid 
in  the  crucible,  the  latter  being  set  in  a  beaker  or  dish. 

F.  .  Alumina.  The  nitrate  from  the  silica  obtained  in 
D  or  ED  combined,  may  be  tested  for  alumina.  It  is 
heated  to  boiling  after  addition  of  a  few  drops  of  nitric 
acid,  and  ammonia  is  added  in  slight  excess.  If  a  white 
or  light-colored,  flocculent,  gelatinous  precipitate  forms, 


118        ROCKS  AND  ROCK  MINERALS 

this  is  aluminium  hydroxide.  If  it  is  reddish  brown,  then 
it  is  wholly  or  in  part  ferric  hydroxide,  indicating  the 
presence  of  iron,  which  is  very  apt  to  be  present  in  silicates, 
especially  colored  ones,  and  the  alumina  may  be  masked. 

If  much  magnesia  is  present  in  the  mineral  its  hydroxide  may 
also  be  precipitated  at  this  point  by  the  ammonia,  unless  the  solutions 
are  rather  diluted  and  a  considerable  quantity  of  ammonium  chloride 
or  nitrate  has  been  formed  by  the  neutralization  of  the  acid  by  the 
ammonia. 

Scrape  some  of  the  precipitate  from  the  filter  paper, 
transfer  it  to  a  clean  test  tube,  or  if  it  is  small  in  amount 
transfer  it  paper  and  all,  and  cover  with  about  5  cubic 
centimeters  of  water;  drop  in  a  piece  of  pure  caustic 
potash,  KOH,  about  the  size  of  a  pea,  and  boil.  The 
alumina,  if  present,  will  go  into  solution  leaving  the  iron 
hydroxide  undissolved;  the  latter  may  be  now  filtered 
off.  Make  the  filtrate  slightly  acid  with  hydrochloric 
acid,  boil  and  then  add  ammonia  in  slight  excess;  alumina, 
if  present,  will  be  precipitated  as  the  white  flocculent 
hydroxide. 

Alumina  can  also  be  detected  before  the  blowpipe  by 
intensely  heating  the  powdered  mineral,  moistened  with 
cobalt  nitrate,  on  charcoal,  when  its  presence  is  indicated 
by  the  mass  turning  blue,  as  mentioned  under  topaz, 
cyanite,  etc.,  in  the  following  tables. 

G.  Iron.  The  detection  of  this  metal  has  been  men- 
tioned above  in  F.  A  more  delicate  method  is  to  add  a 
few  drops  of  potassium  ferrocyanide  solution  to  a  few 
drops  in  water  of  the  final  filtrate  obtained  in  D  or  ED 
combined,  after  boiling  with  a  few  drops  of  nitric  acid  in 
case  hydrochloric  was  originally  used.  The  formation  of 
a  deep  Prussian  blue  precipitate  or  coloration,  if  the 
solutions  are  very  dilute,  indicates  the  presence  of  iron. 
The  nitric  acid  converts  the  ferrous  salt  in  the  solution  into 
a  ferric  one.  Potassium  /erro-cyanide  produces  a  Prussian 
blue  with  ferric  salts,  not  with  ferrous,  while  conversely 


THE   DETERMINATION  OF   ROCK  MINERALS     119 

potassium  /em-cyanide  produces  the  same  effect  with 
ferrous  salts.  Thus  by  testing  portions  of  the  original 
solution  of  the  mineral  in  hydrochloric  acid  with  these  two 
reagents  the  state  or  states  of  oxidation  of  the  iron  in  the 
original  mineral  can  be  ascertained. 

Iron  is  also  shown  when  minerals  become  magnetic 
after  being  heated  in  the  reducing  flame  of  the  blowpipe. 

H.  Calcium.  The  ammoniacal  filtrate  from  the 
hydroxides  of  alumina  and  iron  obtained  in  F,  or  the 
clear  liquid  in  case  ammonia  failed  to  precipitate,  may 
contain  lime  salts  in  solution.  To  prove  the  presence  of 
lime  it  should  be  heated  to  boiling  and  some  ammonium 
oxalate  added,  when  the  formation  of  a  precipitate  of 
oxalate  of  lime  proves  the  presence  of  this  element.  If 
it  should  be  desired  to  further  test  the  solution  for  mag- 
nesia the  lime  oxalate  must  be  removed  by  filtration;  it  is 
allowed  to  stand  for  some  time  and  then  filtered;  if  the 
filtrate  runs  through  turbid  it  should  be  again  passed 
through  the  paper  until  the  liquid  is  clear.  To  this  a 
little  more  ammonium  oxalate  is  added  to  prove  the  com- 
plete precipitation  of  the  h'me. 

I.  Magnesium.  Ordinarily  this  element  should  not 
be  tested  for  until  the  alumina,  iron  and  lime  have  been 
removed,  as  directed  in  F  and  H,  or  their  absence  ascer- 
tained. To  the  solution  thus  -obtained  some  sodium 
phosphate  and  a  considerable  quantity  of  strong  ammonia 
are  added.  The  formation  of  a  precipitate,  ammonium 
magnesium  phosphate,  proves  the  presence  of  this  element. 
If  a  precipitate  does  not  form  at  once  it  is  not,  howyever, 
safe  to  consider  magnesia  absent,  for  if  the  amount  is 
small  and  the  solution  warm  it  may  not  appear  until  the 
liquid  has  become  cold  and  has  stood  for  some  time.  It 
is  then  apt  to  appear  as  a  crystalline  precipitate  attached 
to  the  walls  of  the  vessel. 

J.  Sodium.  A  mineral  containing  sodium  when  heated 
before  the  blowpipe  colors  the  flame  bright  yellow. 
The  best  effect  is  obtained  with  silicates  when  the 


120  ROCKS  AND  ROCK  MINERALS 

powdered  mineral  is  previously  mixed  with  an  equal 
volume  of  powdered  gypsum  and  a  little  of  this  taken  upon 
a  clean  moistened  platinum  wire  which  has  been  previously 
tested.  The  reaction  is,  however,  so  delicate  and  pro- 
duced so  strongly  by  minute  quantities  of  the  element  or 
accidental  traces  that  great  judgment  must  be  used  in 
employing  it.  It  is  only  when  the  coloration  is  very 
intense  and  prolonged  that  the  mineral  should  be  inferred 
to  contain  soda  as  an  essential  oxide. 

K.  Potassium.  This  element  may  be  detected  by  the 
violet  color  it  communicates  to  the  Bunsen  or  blowpipe 
flame.  In  silicates  it  is  best  obtained  by  powdering  the 
mineral  and  mixing  it  with  gypsum,  as  mentioned  under 
sodium  in  J.  The  flame  color  is  delicate  and  entirely 
obscured  by  any  sodium  present;  in  this  case  it  can  be 
seen  by  viewing  it  through  a  piece  of  thick,  dark  blue 
glass  which  cuts  off  all  but  the  potash  flame.  Through 
this  it  will  appear  of  a  violet  or  reddish  purple. 

Another  test  is  to  take  a  small  portion  of  the  final 
filtrate  obtained  in  D  or  ED  combined,  evaporate  it  to  a 
very  small  volume,  add  an  equal  volume  of  alcohol  and 
if  turbid  filter  it.  Then  add  a  few  drops  of  hydrochlor- 
platinic  acid,  B^PtCle,  and  if  a  heavy  yellow  or  orange 
colored  crystalline  precipitate,  potassium  platinic  chloride, 
K^PtCle,  forms  it  shows  the  presence  of  this  element.  No 
ammonium  salt  must  be  present  or  it  will  yield  a  similar 
precipitate. 

L.  Hydrogen  —  Water.  If  a  little  of  the  powdered 
mineral  be  placed  in  a  glass  tube,  one  end  of  which  has 
been  closed  by  fusion  and  drawing  off,  and  gently  heated 
below  redness,  the  evolution  of  water,  which  collects  on  the 
upper  walls  of  the  tube,  shows  that  it  contains  loosely 
attached  water  of  crystallization.  This  occurs  with 
zeolites,  such  as  analcite,  NaAlSi2O6  +  H20,  and  with 
gypsum,  CaSO4  +  2  H2O.  On  the  other  hand,  some 
minerals,  and  many  silicates  are  among  them,  contain 
hydrogen  and  oxygen  firmly  attached  in  the  form  of 


THE  DETERMINATION  OF  ROCK  MINERALS      121 

hydroxyl  —  OH,  and  this  is  only  given  off  as  water  at  a 
very  high  heat.  Indeed  with  some,  as  for  instance 
staurolite  and  talc,  Mg3Si4010(OH)o,  it  is  necessary  to 
subject  the  assay  to  intense  ignition  by  heating  it  white 
hot  before  the  blowpipe  before  the  water  is  given  off. 
This  difference  in  behavior  will  often  serve  as  a  useful 
test  in  determining  minerals.  Many  minerals  which 
contain  hydroxyl  also  contain  fluorine  and  in  this  case  it 
will  be  often  found  that  the  water  evolved  in  the  tube 
gives  an  acid  reaction  to  test  paper  and  the  glass  may  be 
etched.  Unless  the  latter  occurs  the  test  is  not  however 
decisive  of  the  absence  or  presence  of  fluorine. 

M.  Fluorine.  This  is  best  tested  for  as  described  under 
topaz,  on  page  81,  in  a  bulb  tube  with  sodium  meta- 
phosphate. 

N.  Chlorine.  This  occurs  in  rock  salt,  apatite  and 
sodalite.  The  test  is  the  precipitation  of  chlorine  as 
silver  chloride,  AgCl,  in  the  solution  by  addition  of  a  few 
drops  of  silver  nitrate.  The  white  precipitate  turns 
bluish  gray  on  exposure  to  light.  The  test  for  chlorine 
is  very  delicate  and  slight  impurities  may  cause  a  faint 
opalescence  in  the  liquid  on  addition  of  the  silver  salt. 
Rock  salt  is  easily  told  by  its  solubility  in  water,  taste 
and  associations.  Apatite  usually  contains  only  a  very 
little  chlorine  yielding  a  faint  test,  or  chlorine  may  be 
wanting  in  it.  Sodalite  dissolves  in  dilute  nitric  acid  and 
silver  nitrate  produces  in  this  a  considerable  precipitate 
of  the  chloride;  the  nitric  acid  solution  also  yields  gelatin- 
ous silica  as  in  D ;  these  tests  suffice  to  identify  the  mineral. 

0.  Sulphuric  Acid.  Barium  chloride  produces  in  the 
solution  containing  a  sulphate  a  heavy  white  precipitate 
of  barium  sulphate,  BaSO.*,  insoluble  in  hydrochloric  or 
nitric  acid.  Gypsum,  anhydrite  and  noselite  contain 
sulphuric  acid;  they  dissolve  in  hydrochloric  acid  and 
it  may  be  tested  for  as  above.  Noselite  also  yields 
gelatinous  silica,  as  in  D,  and  the  two  reactions  serve  to 
identify  it. 


122  ROCKS  AND  ROCK   MINERALS 

P.  Phosphoric  Acid.  Dissolve  the  powdered  mineral 
(see  A)  in  nitric  acid  and  add  some  solution  of  ammonium 
molybdate,  a  yellow  precipitate  of  ammonium  phospho- 
molybdate  shows  the  presence  of  phosphorus.  This  test 
is  very  delicate.  It  should  be  conducted  with  cold  or 
nearly  cold  solutions.  The  precipitate  is  soluble  in  excess 
of  ammonia.  If  it  is  desired  to  make  this  test  in  a  mixture 
of  minerals,  as  in  a  fine-grained  rock  for  instance,  and 
silica  may  be  in  the  solution,  it  is  best  to  evaporate  the 
latter  and  get  rid  of  the  silica  as  directed  in  D.  The 
phosphoric  acid  can  then  be  tested  for  in  the  filtrate 
acidified  with  nitric  acid.  Apatite  is  the  only  common 
rock-making  mineral  containing  phosphoric  acid,  and  its 
presence  in  rocks  and  soils  can  usually  be  shown  by  this 
test  when  it  cannot  be  detected  megascopically. 

Tables  for  the  Megascopic  Determination  of  Rock  Minerals. 

The  two  following  tables  will  be  found  useful  in  helping 
to  identify  the  commoner  rock-making  minerals.  Beside 
those  given  in  the  tables  there  are  many  less  common 
minerals  which  enter  into  the  composition  of  rocks  and 
which  may  at  times  become  of  local  importance.  This 
is  especially  true  in  metamorphic  limestones  and  schists. 
Some  of  the  more  important  of  them  have  been  given  in 
the  preceding  chapter  on  the  characters  of  minerals,  but 
only  about  fifty  minerals  or  mineral  groups  constituting 
the  kinds  which  are  ordinarily  met  with  in  megascopic 
rock  study  are  here  included.  The  tables  can  only  be 
used  to  distinguish  from  one  another  the  minerals  which 
are  named  in  them;  they  cannot  in  general  be  used  to 
distinguish  them  from  all  other  minerals.  If  doubt  arises 
and  a  mineral  seems  to  be  other  than  any  of  those  described 
here  the  larger  manuals  of  descriptive  and  determinative 
mineralogy  must  be  consulted  for  its  identification. 

Table  1.  This  is  based  solely  on  the  most  obvious  and 
easily  determinable  physical  properties  and  includes  about 
thirty  common  minerals  or  mineral  groups.  It  may 


THE  DETERMINATION  OF  ROCK  MINERALS      123 

often  be  used  to  advantage  in  the  field.  The  only  appa- 
ratus required  in  its  use  are  a  lens,  pocket  knife  and  frag- 
ments of  quartz  and  feldspar,  in  addition  to  the  hammers 
usually  carried.  It  will  be  of  advantage  to  have  one 
blade  of  the  knife  magnetized.  The  streak  or  color  of  the 
powdered  mineral  can  be  tested  by  grinding  a  small  piece 
to  powder  between  two  hammer  faces,  pouring  it  on  a 
piece  of  white  paper  and  rubbing  the  dust  with  the  finger 
to  observe  the  color  produced.  A  piece  may  be  cracked 
into  smaller  grains  and  these  examined  with  the  lens  to 
observe  the  cleavage  if  it  is  not  well  shown  by  the  mineral 
on  the  fractured  rock  surface.  The  transparency  or 
translucency,  if  not  obvious  in  the  mineral  in  the  rock, 
may  be  tested  by  holding  a  fragment  or  sliver  against  the 
light  and  observing  if  light  is  transmitted  through  its 
thinnest  edges.  The  hardness  is  best  tested  on  a  smooth 
lustrous  cleavage  face  with  the  knife  point  or  a  sharp- 
pointed  fragment  of  quartz  or  feldspar,  substances  which 
are  usually  readily  obtainable. 

Table  2.  This  includes  about  fifty  of  the  prominent 
rock  minerals  or  mineral  groups  whose  characters  are 
treated  in  the  foregoing  descriptive  portion.  It  requires 
for  its  use  some  of  the  simpler  apparatus  and  reagents 
found  in  every  chemical  and  mineralogical  laboratory 
and  the  knowledge  of  how  to  use  them.  They  have  been 
already  mentioned  on  page  12. 

The  table  is  based  upon  those  of  the  Brush-Penfield 
Determinative  Mineralogy  which  have  been  modified  to 
meet  the  demands  of  this  particular  place,  and  if  further 
information  is  desired  that  manual  may  be  consulted  to 
advantage. 

This  table  is  much  more  complete  and  certain  in  its 
identification  than  Table  1  and  should  always  be  used 
in  preference  to  it  when  possible.  Table  1  is  to  be 
considered  a  more  or  less  rough  method  of  approximation 
to  be  used  in  the  field  or  when  no  apparatus  or  reagents 
are  at  hand. 


124  ROCKS  AND  ROCK  MINERALS 

It  should  be  again  repeated  that  the  table  cannot  be 
used  for  the  identification  of  all  minerals  which  occur  in 
rocks  but  only  to  distinguish  the  commoner  ones,  men- 
tioned in  it,  from  one  another.  In  most  cases  the  identi- 
fication of  the  mineral  is  complete,  but  instances  may 
occur  where  some  comparatively  rare  one  will  give  similar 
reactions.  Thus  the  rare  mineral  aragonite  would  lead  to 
the  same  place  as  calcite,  but  reference  to  the  description 
of  the  latter  would  show  at  once  that  it  differs  markedly 
in  other  properties,  such  as  cleavage  and  crystallization. 
This  will  be  usually  found  to  be  the  case,  and  if  further 
information  is  desired  it  must  be  sought  elsewhere.  But 
within  the  limits  imposed  the  table  should  serve  a  useful 
purpose  to  the  student  of  rocks. 

TABLE  1. 

The  mineral  has  a  fine  cleavage  in  one  direction;  is  sometimes 
micaceous  and  may  be  split  into  thin  leaves  by  the  use  of  the 
knife  point.  Sec.  1  below. 

Has  a  good  cleavage  in  two  directions.     Sec.  2. 

Has  a  good  cleavage  in  three  directions,  forming  cubes  or 
rhombs.  Sec.  3. 

Has  a  fine  fibrous  structure  and  cleavage.     Sec.  4. 

No  apparent  good  cleavage.     Sec.  5. 

SEC.  1.     Cleavage  in  one  direction. 

A.  Micaceous.     Cleavage   leaves   tough,    flexible,    elastic.     Occurs 

in  crystals,  shreds,  flakes.  Black,  brown,  gray  or  white. 
Transparent-translucent.  Mica,  p.  50. 

B.  Micaceous.     Cleavage    leaves    tough,    flexible,    non-elastic.     In 

crystals,  shreds,  masses.  Usually  green  to  dark  green. 
Chlorite,  p.  98. 

C.  Often  micaceous.     Leaves  flexible  but  non-elastic.     Greasy  feel, 

very  soft,  marks  cloth.  White,  greenish,  gray.  Usually  in 
foliated  masses.  Translucent.  Talc,  p.  102. 

D.  Leaves  somewhat  flexible  but  showing  cross  cleavage  cracks 

when  bent;  in  one  direction  fibrous,  the  other  brittle  forming 
rhombs.  Soft,  scratched  by  finger  nail,  but  not  greasy  in  feel. 
Usually  colorless,  white  or  reddish;  transparent  to  trans- 
lucent. In  crystals,  masses,  seams.  Gypsum,  p.  111. 

E.  Leaves  have  a  brilliant  metallic   luster,   like  polished   steel. 

Hematite  (micaceous  variety)  p.  91. 


THE  DETERMINATION  OF  ROCK  MINERALS     125 

F.  Leaves  brittle;  lozenge  shaped  outline.     Usually  white,  trans- 

lucent. Scratched  by  the  knife.  Crystals  in  cavities. 
Heulandite,  p.  104. 

G.  Not  micaceous,  massive,  brittle.     Very  hard,  not  scratched  by 

knife  or  feldspar.  Yellowish  green  to  dark  green,  translucent. 
In  crystals  or  masses.  Epidote,  p.  73. 

SEC.  2.     Cleavage  in  two  directions. 

A.  Two  cleavages  at  or  very  nearly  at  90  degrees.     Brittle,  hard, 

not  scratched  by  knife  but  by  quartz.  Usually  of  a  light 
color,  white,  pink  to  red  or  gray,  translucent.  In  crystals, 
grains,  masses.  Feldspar,  p.  34. 

B.  Usually  of  a  dark  color,  greenish  to  black;  in  grains  or  short 

prisms;  sometimes  light  colored  in  metamorphic  rocks  and 
then  often  elongated  columnar  in  cleavage  direction.  Cleav- 
age good  but  not  eminent;  prismatic.  Cleavage  angles  87 
and  93  degrees.  Usually  scratched  by  feldspar.  Pyroxene, 
p.  55. 

C.  Usually  of  a  dark  color,  greenish  to  black.     Apt  to  be  in  crystals 

elongated  or  bladed  in  cleavage  direction.  Sometimes  light 
colored  in  metamorphic  rocks.  Cleavage  very  good  with 
shining  surface.  Cleavage  angles  55  and  125  degrees. 
Usually  scratched  by  feldspar.  Amphibole,  p.  60. 

SEC.  3.     Cleavage  in  three  directions. 

A.  Cleavages  not  at  right  angles,  forming  rhombs.     Easily  scratched 

by  knife.  Usually  white,  sometimes  tinted  various  shades 
to  black;  transparent  to  translucent.  In  crystals,  masses, 
veins,  etc.  Calcite,  p.  105,  or  Dolomite,  p.  108.  (If  rhombic  sur- 
faces of  crystals  are  curved,  probably  dolomite.) 

B.  Cleavages  at  right  angles  forming  cubes,  soluble,  strong  saline 

taste.  Transparent  colorless  or  white,  rarely  tinted.  In 
crystalline  masses.  Halite,  rock  salt,  p.  113. 

C.  Cleavage  apparently  as  above.     No  perceptible  taste.     Easily 

scratched  by  the  knife.  White,  bluish.  In  crystalline 
masses.  Anhydrite,  p.  113. 

D.  Apparent    cleavages    sometimes    forming    rhombs,    sometimes 

apparently  cubic.  Very  hard,  scratches  quartz  easily.  In 
hexagonal  crystals,  grains  or  lumps  of  a  dark,  smoky,  or 
bluish  gray;  more  or  less  translucent.  Corundum,  p.  86. 

SEC.  4.     Has  a  fine  fibrous  or  columnar  structure 

A.  In    opaque    brown    to    black    masses.     Streak    yellow-brown. 

Limonite,  p.  93. 

B.  In  opaque  red-brown  to  black  masses.     Streak  brownish  red. 

Hematite,  p.  91. 


126  ROCKS  AND  ROCK  MINERALS 

C.  White  or  reddish ;  translucent.     Brittle.     Often  radially  fibrous. 

Sometimes  showing  slender  prismatic  crystals.  Difficultly 
scratched  by  the  knife.  Occurs  in  cavities,  veins  or  seams. 
Natrolite,  p.  103. 

D.  White  or  reddish ;  translucent  or  transparent.  Brittle.  Often  radi- 

ally fibrous.  Compound  crystals  often  sheaf  shaped.  Scratched 
by  knife.  In  cavities,  veins  or  seams.  Stilbite,  p.  103. 

E.  Shreds  easily  into  fine,  flexible  fibers  like  cotton  or  silk.     White 

or  light  gray.  a.  Hornblende  asbestus,  page  65.  b.  White 
to  yellowish  brown;  silky;  generally  in  veins  in  or  associated 
with  serpentine.  Chrysotile  (serpentine)  asbestus,  page  101. 

F.  White  or  pale  colors.     Translucent.     Brittle.     Easily  scratched 

by  knife  but  not  by  finger  nail.     In  masses.     Calcite,  p.  105. 

G.  White  to  pale  red.     Silky  luster,  translucent.     Brittle,  soft, 

scratched  by  finger  nail.  In  masses  and  seams.  Gypsum, 
p. 111. 

SEC.  5.     Without  good  or  apparent  cleavage 

A.  Opaque,     brass-yellow    crystals    with    metallic    luster.     Not 

scratched  by  the  knife.     Pyrite,  p.  94. 

B.  Opaque,  earthy,  brown  to  brown-black  masses.     Streak  yellow- 

brown.     Scratched  by  the  knife.     Limonite,  p.  93. 

C.  Opaque,  reddish  brown  to  black  masses,  or  crystals  and  grains, 

iron  black  with  metallic  luster.  Streak  brownish  red. 
Scratched  by  the  knife.  Hematite,  p.  91. 

D.  Opaque,  iron  black  masses,  grains  or  octahedrons  with  metallic 

luster.  Streak  black.  Magnetic.  Not  scratched  by  the 
knife.  Magnetite,  p.  89. 

E.  Opaque,  black  grains  or  masses  often  with  reddish  tone.     Luster 

metallic  to  submetallic.  Streak  black  to  reddish  black.  Not 
noticeably  magnetic.  Scarcely  or  not  scratched  by  the  knife. 
Ilmenite,  p.  90. 

F.  In  garnet-shaped  crystals  or  spherical.     Usually  dark  red  to 

black  and  translucent.  Brittle.  Not  scratched  by  feldspar. 
Garnet,  p.  70. 

G.  In  garnet-shaped  crystals.     Colorless  or  white  to  gray  white, 

translucent.  Not  scratched  by  knife  but  by  feldspar.  Leucite, 
p.  49,  or  Analcite,  p.  103. 

H.  In  transparent  to  translucent  crystals  or  grains  of  a  light  yellow- 
ish- or  bottle-green  color.  Not  scratched  by  feldspar. 
Olivine,  p.  67. 

/.  In  prismatic  crystals,  generally  slender,  shiny  and  black  with 
triangular  cross  section.  Not  scratched  by  quartz.  Tour- 
maline, p.  78. 


THE  DETERMINATION  OF  ROCK  MINERALS      127 

/.    In   grains,    masses    or   hexagonal,    pyramidal    crystals.     Con- 

choidal  fracture.     Greasy  to  glassy  luster.     Colorless,  white, 

smoky,  dark;  transparent  to  translucent.     Not  scratched  by 

feldspar.      Quartz,  p.  83. 
K.  In  grains  or  masses,   rarely  in  crystals  with   rectangular  or 

hexagonal     sections.     Conchoidal     fracture.     Greasy,     oily 

luster.     White,  gray  or  reddish;  translucent.     Scratched  by 

feldspar.     Nephelite,  p.  47. 
L.    In  grains  or  masses,  generally  of  a  bright  blue  color.     Sodalite, 

p.  48. 
M .  In  masses,  of  a  dark  or  yellowish  green,  easily  scratched  or  cut 

by  knife.     Serpentine,  p.  100. 
N.   In  masses,  often  somewhat  foliated.     Greasy  feel;  very  soft, 

marks  cloth.     White,  greenish,  gray.     Talc,  p.  102. 
O.    In  hexagonal  crystals,  grains  or  lumps.     Dark  smoky  or  bluish 

gray;   translucent.     Very   hard,    not   scratched   by   quartz, 

garnet  or  tourmaline.     Corundum,  p.  86. 
P.    In  masses,  compact  or  chalky.     Friable,  very  soft,  easily  cut  by 

finger  nail.     Rubbed  between  the  fingers  has  a  soft  soapy  feel. 

Kaolin,  p.  96. 

TABLE  2 

A.  The  mineral  has  a  metallic  luster  or  is  opaque  and  gives  a 

dark  or  strongly  colored  streak.     2.* 

B.  The  mineral  is  without  metallic  luster  or  is  transparent  or 

translucent  on  very  thin  edges  and  its  streak  is  white  or 
light-colored.     6.* 

A.  Heated  in  the  blowpipe  flame  the  mineral  burns  and  gives 

off  sulphurous  fumes.     Has  a  brass  yellow  color.     Pyrite, 
p.  94. 

B.  Heated  in  the  reducing  blowpipe  flame  becomes  magnetic 

when   cold.     Not   brassy   in    appearance.     Infusible   or 
very  difficultly  so.     Iron  oxides.     3. 

I  A.   Is    magnetic    without    heating.     Magnetite    (and    in    part 
Ilmenite"),  p.  89. 
B.    Is  only  magnetic  after  heating.     4. 

{A.   Heated  in  the  closed  glass  tube  gives  water.     Limonite,  p.  93. 
B    Gives  little  or  no  water.     5. 

I  A.   Reacts  for  titanium.     Ilmenite,  p.  90. 
B.    No  reaction  for  titanium.     Streak  brownish  or  Indian  red. 
Hematite,  p.  91. 

a,  |  A.   Fusible  before  the  blowpipe  (fusibility  1-5).    7. 
1 1  B.   Infusible  or  very  difficultly  fusible.     17. 

*  The  appended  number  in  each  case  refers  to  that  in  front  of  a  succeeding 
section. 


128  ROCKS  AND  ROCK  MINERALS 

A.  Become   magnetic   after  heating   before   the   blowpipe  in 

reducing  flame.     8. 

B.  Do  not  become  magnetic.     11. 

A.  Soluble  in  hydrochloric  acid  with  separation  of  silica,  some- 

times gelatinous.     9. 

B.  Insoluble  in  hydrochloric  acid  or  only  slightly  acted  on.     10. 

A.  Micaceous  or  foliated.     Mica  (Biotite  or  Lepidomelane), 

p.  50. 

B.  Isometric      crystals.       Gelatinizes      imperfectly.       Garnet 

(Andradite),  p.  70. 

C.  Gelatinizes.     Olivine,  rich  in  iron  —  Fayalite,  p.  67. 

A.  Micaceous  —  difficultly  fusible.     Biotite,  p.  50. 

B.  Isometric    crystals    or   spherical    in   shape.     After   fusion 

gelatinizes  with  HC1.  Dark  red  color.  Garnet  (Alman- 
dite),  p.  70. 

C.  Quietly  and  difficultly  fusible.     Greenish  black  or  bronze- 

brown.     Good  cleavage.     Pyroxene  (Hypersthene),  p.  55. 

D.  Fuses   with    intumescence   coloring   flame    strong   yellow. 

Black.  Prismatic  cleavage,  angle  55  degrees.  Amphi- 
bole  (Arfvedsonite),  p.  60. 

E.  Fuses    quietly,   coloring    flame    yellow.     Black,   prismatic 

cleavage,  93  degrees.     Pyroxene  (Aegirite),  p.  55. 

A.  Readily  and  completely  soluble  in  water;  has  a  saline  taste. 

Halite,  rock-salt,  p.  113. 

B.  Difficultly  soluble  in  water.     After  intense  ignition  colors 

moistened  turmeric  paper  brown. 

a.  Gives  much  water  in  closed  glass  tube,  Gypsum,  p.  1 1 1 . 
6.  Gives  little  or  no  water  in  closed  tube,  Anhydrite. 
p.  113. 

C.  Soluble  in  hydrochloric  acid  without  gelatinizing  or  separa- 

tion of  silica  on  evaporation.  A  drop  of  sulphuric  acid 
in  the  solution  precipitates  calcium  sulphate.  Apatite, 
p.  95. 

D.  Soluble  in  hydrochloric  acid  with  gelatinization. 

a.  Heated  in  closed  glass  tube  gives  off  water.     12. 

b.  Heated  as  above  yields  little  or  no  water.     13. 

E.  Soluble  in  hydrochloric  acid,  silica  separates  but  no  jelly 

forms. 

a.  Heated  in  closed  glass  tube  gives  off  water.     14. 
6.  Heated  as  above  yields  little  or  no  water.     15. 

F.  Insoluble  in  hydrochloric  acid.     16. 

A.  Fuses    quietly    to    a    clear   transparent    glass.     White   or 

colorless ;  in  slender  crystals  or  fibrous  bunches.  Natrolite, 
see  Zeolites,  p.  103. 

B.  A  fragment  in  warm   dilute  hydrochloric   acid   gives  off 

minute  bubbles  of  CO2  gas.  Cancrinite,  see  Felds- 
pathoids,  p.  48. 


THE  DETERMINATION  OF  ROCK  MINERALS     129 


A. 


Fuse  rather  easily 
before  the  blow- 
pipe, coloring 
the  flame  strong 
yellow.  Dissolve 
easily  in  very 
dilute  nitric 
acid  and  gela- 
tinize. 


a.  The  nitric  acid  solution  gives  a  pre- 
cipitate with  silver  nitrate  solution 
(page  121).  Color  usually  blue. 
Sodalite,  p.  48. 

6.  The  nitric  acid  solution  gives  a  pre- 
cipitate with  barium  chloride  solu- 
tion. Hauynite-Noselite,  p.  48. 

c.  No  reaction  with  silver  nitrate  or 
barium  chloride.  Nephelite.  p.  47. 


[Difficultly  soluble  in  hydrochloric  acid  and  colors  the  flame 
B.\     very  little.     Has  a  good  cleavage  in  two  directions.    Anor- 
|     thite,  see  Feldspars,  p.  34. 

A.  Usually  in  greenish  masses,   compact,  greasy,  sometimes 

fibrous.     Difficultly  fusible.     Serpentine,  p.  100. 

B.  Fuses  quietly  to  a  clear  glass  coloring  flame  yellow.     Gen- 

erally in  colorless  or  white  garnet-like  crystals.  Analcite, 
p.  103. 

C.  Fuses    with    swelling    and    intumescence.     Commonly    in 

sheaf-like  or  radiated  crystals.     Stilbite,  p.  103. 

D.  Fuses  as  in  (7.     Crystals  have  a  fine  cleavage  with  pearly 

luster  and  lozenge-shaped  section.     Heulandite,  p.  104. 

A.  Fuses  quietly  to  a  glassy  globule.     Slowly  acted  on  by 

hydrochloric  acid.  Good  cleavage  in  two  directions;  one 
generally  shows  fine  parallel  twinning  lines.  Often 

frayish  or  bluish  with  a  play  of  colors.     Labradorite,  see 
eldspars,  p.  34. 

B.  Fuses  quietly  to  white  globule.     Easily  soluble  in  hydro- 

chloric acid;  solution  evaporated  to  dryness,  residue 
moistened  with  little  hydrochloric  and  dissolved  in  water 
and  filtered,  ammonia  produces  little  or  no  precipitate. 
Wollastonite,  CaSiOg,  a  variety  of  Pyroxene,  generally  of 
a  white  color. 

A.  Micaceous.     Cleave  into  thin  flexible  elastic  plates  in  one 

direction.     Micas,  p.  50. 

B.  Micaceous.     Cleaves    into    thin    plates,    flexible    but    not 

elastic,  micaceous.  Very  soft  and  has  a  greasy  feel. 
Color  white,  gray  or  greenish.  Talc,  p.  102. 

C.  Cleavable,  micaceous,  but  cleavage  plates  not  elastic,  though 

flexible.  Soft,  but  not  so  soft  as  talc.  Color  green, 
usually  rather  dark  green.  Chlorite,  p.  98. 

D.  Not  micaceous.     Solid  and  brittle.     Good  cleavage  in  two 

directions  at  or  about  90  degrees.  Generally  light 
colored,  red  or  gray.  Hard,  cannot  be  scratched  by 
knife.  Difficultly  fusible.  Feldspar,  p.  34. 
E.  Before  the  blowpipe  fuses  with  swelling  and  bubbling. 
Very  hard,  scratches  feldspar.  Generally  in  black 
columnar  crystals,  sometimes  red  or  green.  No  cleavage. 
Tourmaline,  p.  78. 


17 


130  ROCKS  AND  ROCK  MINERALS 

F.  Fuses  quietly.     Gelatinizes   with   hydrochloric   acid  aftei 

fusion.  Crystals  as  on  page  70  or  in  spherical  forms. 
Very  hard.  No  good  cleavage.  Garnet,  p.  70. 

G.  Fuses  with  swelling  and  intumescence  to  a  black  slaggy 

mass  which  gelatinizes  in  hydrochloric  acid.  Powdered 
mineral  on  intense  heating  in  closed  glass  tube  yields  a 
little  water.  Yellowish  to  blackish  green.  Epidote,  p.  73. 
,  H.  Fuses  quietly  or  with  little  intumescence.  Generally 
scratched  by  feldspar. 

a.  Prismatic  cleavage  with  angle  of  87  degrees.   Pyroxene 

p.  55. 
6.  Prismatic  cleavage  with  angle  of  55  degrees.     Amphi- 

bole,  p.  60. 

/.  Fuses  with  intumescence  to  a  greenish  or  brownish  glass 
which  will  gelatinize  with  hydrochloric  acid.  Vesuvianite, 
p.  75. 

A.  After  intense  ignition  before  the  blowpipe  gives  a  brown 

stain  when  placed  on  moistened  turmeric  paper.     18. 

B.  Dissolves  in  hydrochloric  acid  but  without  gelatinizing  or 

yielding  a  residue  of  silica  on  evaporation.     19. 

C.  a.  Dissolves  in  hydrochloric  acid  and  gelatinizes.     Olivine, 

p.  67. 
b.  Reacts  for  fluorine  (see  topaz  22  F.).     Chondrodite,  p.  82. 

D.  Dissolves   in   hydrochloric   acid,    does   not   gelatinize   but 

silica  separates.     20. 

E.  Insoluble  in  hydrochloric  acid. 

a.  Can  be  scratched  by  glass  or  a  knife  point.     21. 
ft.  Cannot  be  scratched  by  glass  or  the  knife.     22. 

I  A.   Effervesces  freely  in  cold  dilute  acid.     Calcite,  p.  105. 
B.    Effervesces  freely  in  hot  but  not  in  cold  acid.     Dolomite, 
p.  108. 

A.  Heated  in  the  reducing  blowpipe  flame  becomes  magnetic. 

a.  Little  or  no  water  in  closed  tube;  streak  brown-red. 

Hematite,  p.  91. 
6.  Water  in  closed  glass  tube;  streak  yellow-brown. 

Limonite,  p.  93. 

B.  In  hexagonal  crystals  usually.     Gives  reactions  for  phos- 

phorus. A  little  dilute  sulphuric  acid  gives  a  precipitate 
of  white  CaSO4,  in  the  cold  concentrated  solution  of 
mineral  in  hydrochloric  acid.  Readily  scratched  by  the 
knife.  Apatite,  p.  95. 

A.  Commonly  in  compact  green  masses.     Sometimes  fibrous. 

like  asbestus,  then  white  or  brownish  or  yellowish.  Greasy 
feeling,  easily  scratched  by  knife.  Serpentine,  p.  100. 

B.  In  spherical   or  garnet-shaped  crystals.     White  to  gray 

Leucite,  p.  49. 


19 


20 


21 


22  - 


THE  DETERMINATION  OF  ROCK  MINERALS     131 

A.  Micaceous.     Cleavage    leaves    tough    and    elastic.     Micas, 

p.  50. 

B.  Micaceous.     Cleavage  leaves  tough   and  flexible  but  not 

elastic.  Intense  ignition  in  closed  tube  gives  water. 
Color  green.  Chlorite,  p.  98. 

C.  Very  soft  and  has  a  greasy  feeling.     Talc,  p.  102. 

D.  Clay-like,  compact  or  mealy.     Leaves  undissolved  silica  in 

the  phosphorus  salt  bead.  Gives  water  in  the  closed 
glass  tube.  Kaolin,  p.  96. 

A.  Extremely    hard.     Scratches    quartz.     Generally    has     a 

parting  that  looks  like  cleavage.     Corundum,  p.  86. 

B.  No    cleavage;    conchoidal    fracture.     Scratches    feldspar. 

Sometimes  in  hexagonal  crystals  with  pyramid  at  end. 
Quartz,  p.  83. 

C.  Prismatic  cleavage.     Becomes  black  before  the  blowpipe 

and  very  fine  splinters  fuse  with  difficulty.  Brown  to 
green  or  greenish  black.  Pyroxene  (enstatite-hypers- 
thene),  p.  55. 

D.  Good  cleavage  in  two  directions  at  90  degrees  or  nearly  so. 

Generally  light  in  color,  red  or  gray.  Scratched  by 
quartz.  Fusibility  =  5.  Feldspars,  p.  34. 

E.  In  prismatic  crystals,  often  twinned;  scratches  quartz;  red- 

brown  to  brownish  black;  intense  ignition  in  closed  tube 
gives  a  little  water.  Staurolite,  p.  76. 

F.  Reaction  for  fluorine  when  heated  in  tube  with  soda  meta- 

phosphate.  With  cobalt  nitrate  reacts  for  alumina  (see  G 
below).  One  good  cleavage.  Scratches  quartz.  Topaz, 
p.  81. 

G.  Powdered    mineral    moistened    with    cobalt    nitrate    and 

intensely  heated  by  the  blowpipe  on  charcoal  becomes  blue 
(alumina)',  a,  in  stout  rectangular  prisms,  often  full  of 
impurities,  not  scratched  by  knife.  Andalusite,  p.  77 ;b, 
in  bladed,  generally  blue,  crystals;  scratched  by  knife 
parallel  to  cleavage,  but  not  at  right  angles  to  it.  Cyanite, 
p.  78. 

H.  No  crystal  form  or  structure.  Effervesces  in  Na2CO3  bead. 
Yields  a  little  water  in  closed  tube  on  intense  ignition. 
Opal,  etc.,  p.  86. 


PART  III. 

ROCKS. 


CHAPTER  VI. 
GENERAL  PETROLOGY  OF  IGNEOUS  ROCKS. 

IT  has  been  previously  stated  that  all  rocks  may  be 
divided  into  three  great  natural  groups,  the  igneous,  the 
sedimentary  and  the  metamorphic.  The  igneous  are 
those  which  have  been  formed  by  the  solidification  of 
molten  masses  from  within  the  earth.  With  reference  to 
their  origin  they  have  also  at  times  been  called  the  primary 
rocks  because  the  material  which  composes  the  other  two 
classes  has  been  originally  derived  from  igneous  rocks 
which,  from  time  to  time,  have  been  formed  either  in  or  on 
the  upper  part  of  the  earth's  crust  or  from  the  earth's 
original  crust  itself.  And  if  we  follow  the  view  that  the 
earth  was  once  molten,  the  original  cooling  crust  must 
have  been  of  the  nature  of  igneous  rock.  Hence  in  this 
sense  the  igneous  rocks  are  the  primary  ones. 

Distinguishing  Characters  of  Igneous  Rocks.  The  char- 
acters of  the  igneous  rocks,  by  which  they  may  be 
distinguished  from  the  sedimentary  and  metamorphic 
ones,  are  of  two  kinds  —  field  characters  and  specimen 
characters.  The  field  characters  are  those  which  can  only 
be  observed  in  the  field  by  studying  the  mass  of  rock  in 
its  relation  to  surrounding  masses,  or  in  other  words  its 
mode  of  occurrence,  that  is,  whether  it  is  a  dike,  a  laccolith, 
a  lava  sheet,  etc.  These  modes  of  occurrence  will  be 
presently  described.  If  that  of  a  given  rock  mass  can  be 

132 


GENERAL  PETROLOGY  OF  IGNEOUS  ROCKS     133 

clearly  determined  it  indicates,  more  definitely  than  any- 
thing else,  if  it  is  igneous  in  origin  or  not. 

But  very  often  it  happens  that  the  boundaries  of  a  rock 
mass  are  so  covered  or  obscured  that  its  relations  to  the 
surrounding  rocks  and  its  mode  of  occurrence  cannot  be 
told,  or  it  is  often  necessary  to  determine  the  nature  of  a 
specimen  which  has  been  removed  from  a  parent  mass 
which  is  not  accessible  for  study.  In  this  case  we  are 
compelled  to  fall  back  upon  those  characters  of  the  rock 
which  are  inherent,  and  to  be  observed  by  an  examination 
of  the  material  of  the  outcrop  or  specimen.  Of  these 
there  are  three  principal  ones  which  distinguish  the 
igneous  from  the  sedimentary  and  metamorphic  rocks. 
They  are: 

a.  Entire  absence  of  fossils. 

b.  The  material  composition. 

c.  The  arrangement  of  the  material,  texture  or  structure. 

The  first  character  is  an  obvious  one,  but  it  is  largely  of 
negative  value  since  many  sedimentary  and  most  meta- 
morphic rocks  do  not  contain  fossils. 

The  second  refers  to  whether  the  rock  contains  glass 
or  is  wholly  made  up  of  mineral  grains,  and  if  the  latter, 
the  kinds  and  relative  amounts  of  the  minerals  present. 
If  a  rock  is  made  up  wholly  or  in  part  of  glass  it  is  cer- 
tainly of  igneous  origin.  The  presence  of  certain  minerals 
is  also  proof  of  igneous  origin,  but  no  general  rule  by  which 
a  rock  may  be  certainly  stated  to  be  of  igneous  origin  from 
its  mineral  composition  can  be  given.  This  would  have 
to  be  done  from  a  knowledge  of  the  different  kinds  of 
igneous  rocks  themselves,  as  they  are  described  in  a  fol- 
lowing chapter.  The  third  character  is  that  the  igneous 
rocks  present  a  homogeneous  appearance;  that  a  surface 
of  the  rock  in  one  direction  is  like  a  surface  in  any  other 
direction  —  that  they  do  not  show  the  stratified,  barfded, 
or  foliated  structures  which  are  characteristic  of  the 
sedimentary  and  metamorphic  rocks.  In  addition  there 


134  ROCKS  AND   ROCK  MINERALS 

are  certain  minor  structures  which  sometimes  appear  in 
igneous  rocks,  such  as  the  amygdaloidal,  which  are 
highly  characteristic  and  will  be  described  later. 

There  are  exceptions  to  the  rules  given  above  in  a  and  c,  but  at 
.the  outset  it  is  better  for  the  student  to  consider  them  as  if  absolute, 
and  the  exceptions,  which  will  be  discussed  in  their  appropriate 
places,  will  take  care  of  themselves. 

Occurrences  of  Igneous  Rocks. 

There  are  two  chief  modes  of  occurrence  of  igneous 
rocks  recognized  by  geologists,  the  extrusive  and  the 
intrusive.  In  the  extrusive  the  molten  mass  or  magma 
rising  from  depths  below  has  attained  the  surface,  come 
out  upon  it,  solidified  and  formed  the  rock.  They  are 
also  called  effusive  and  sometimes  volcanic  rocks,  though 
they  are  not  always  connected  with  volcanoes.  In  the 
case  of  the  intrusive  rocks  the  magma  has  stopped  before 
attaining  the  surface  and  has  cooled  and  solidified,  sur- 
rounded by  other  rock  masses  of  the  earth's  upper  crust. 
Each  of  these  cases  has  a  number  of  recognized  sub- 
divisions; with  the  extrusive  rocks  depending  on  the 
conditions  under  which  the  magma  was  ejected  and  with 
the  intrusive  rocks  on  the  relation  which  the  mass  bears 
to  the  rocks  which  surround  it.  Following  the  course  of 
the  magma  upward  we  will  describe  first  the  intrusive 
and  then  the  extrusive  modes  of  occurrence. 

Intrusive  Modes  of  Occurrence.  These  are  dikes, 
sheets,  laccoliths,  necks,  stocks  and  bathyliths.  Various 
other  modes  have  been  recognized  and  described,  but  for 
simplicity's  sake  they  may  be  regarded  as  modifications 
of  these  which  have  just  been  mentioned.  The  simplest 
form  of  intrusion  is  that  of  the  dike,  and  this  will  be 
described  first. 

Dikes.  A  dike  is  the  result  of  the  simple  filling  of  a 
fissure  in  rock  masses  by  molten  magma  from  below 
which  there  solidified.  In  shape,  its  extension  in  length 
and  breadth  is  great  as  compared  with  its  thickness.  It 


PLATE    2. 


03    •< 


GENERAL  PETROLOGY  OF  IGNEOUS  ROCKS      135 

may  "  cut,  "  that  is,  pass  through,  other  igneous  rocks  or 
through  sedimentary  or  metamorphic  ones,  whatever  the 
material  was  in  which  the  fissure  was  formed.  In  passing 
through  sedimentary  rocks  it  always  cuts  at  some  angle 
across  the  planes  of  stratification;  if  parallel  to  them  it 
becomes  an  intrusive  sheet.  A  dike  may  be  of  all  sizes 
from  a  fraction  of  an  inch  in  thickness  up  to  half  a  mile; 
from  two  or  three  feet  up  to  twenty  are  the  ones  most 
commonly  observed;  it  may  be  but  a  yard  or  two  long  as 
exposed  on  the  surface,  or  it  may  be  many  miles;  a  great 
dike  in  the  north  of  England  has  been  traced  over  a 
hundred  miles.  The  plane  of  extension  of  a  dike  in  most 
cases  appears  to  be  vertical  or  nearly  so;  often  it  is  inclined 
at  varying  angles  to  the  vertical  plane.  This  angle  of 
inclination  is  called  the  hade  of  the  dike,  and  the  direction 
which  its  outcrop  takes  in  intersecting  the  horizontal 
plane  is  called  its  trend.  Dikes  may  have  attained  the 
surface  and  given  rise  to  lava  outflows,  or  they  may  not 
and  have  been  exposed  only  by  subsequent  erosion.  In 
the  processes  of  erosion  they  may  have  resisted  better 
than  the  surrounding  rock  and  thus  project  as  walls,  a 
common  feature;  or  they  may  have  resisted  less  well  and 
have  become  ditches,  which  is  less  common.  Dikes  very 
often  show  pronouncedly  the  columnar  structure  described 
later,  the  columns  lying  at  right  angles  to  the  walls. 
Where  dikes  have  cut  through  sedimentary  rocks  they 
have  often  changed  and  altered  them  for  some  distance  in 
the  manner  described  under  contact  metamorphism.  A 
view  of  a  dike  cutting  a  sheet  of  igneous  rock  and  stratified 
beds  is  seen  in  Plate  2. 

Intrusive  Sheets.  It  frequently  happens  that  where 
molten  magma  is  being  forced  upward  through,  or  into, 
stratified  rocks,  that  it  attains  a  place  where  the  con- 
ditions are  such  that  it  is  easiest  for  it  to  spread  out  in  a 
layer  between  the  sedimentary  beds.  This  frequently 
happens  where  the  beds  are  weak  and  easily  penetrated, 
as  in  shales,  thinly  bedded  sandstones,  etc.  The  form  of 


136  ROCKS  AND  ROCK  MINERALS 

such  a  mass  is  like  that  of  a  dike,  but  unlike  the  latter  it 
lies  concordantly  along  the  planes  of  stratification.  Such 
sheets  may  be  only  a  foot  or  less  in  thickness,  and  from 
this  up  to  several  hundred  feet  or  even  more:  they  may 
spread  over  many  miles  in  extent.  Like  dikes  they  often 
show  a  columnar  structure,  the  columns  being  perpen- 
dicular to  the  upper  and  lower  surfaces  and  thus  often 
vertical.  Sometimes  they  break,  dike-like,  across  the 
strata  and  are  continued  along  a  new  horizon.  They  are 
usually  of  the  same  hard,  firm  rock  at  top  and  bottom, 
and  to  some  extent  have  altered  and  changed  the  sedi- 
mentary beds  both  above  and  below  them:  these  char- 
acters distinguish  them  from  surface  flows  of  lavas  which 
have  been  buried  by  later  deposits  of  sediment  upon 
them.  They  are  most  apt  to  occur  in  connection  with 
larger  and  more  important  intrusions  of  magma,  such  as 
stocks,  laccoliths,  etc.,  as  accompanying  or  dependent 
features.  In  regions  where  thick  extensive  sheets  occur 
and  the  strata  have  been  dislocated,  faulted,  and  upturned 
they  often  give  rise,  through  erosion,  to  prominent  topo- 
graphic features  as  illustrated  in  the  trap  ridges  of  southern 
New  England,  northern  New  Jersey,  and  in  Scotland  at 
Edinburgh  and  in  many  other  places.  In  Great  Britain 
and  frequently  in  Canada  an  intrusive  sheet  is  called  a 
sill.  Examples  are  shown  on  Plates  2  and  3. 

Laccoliths.  These  are  great  lenticular  masses  of  igneous 
rock  lying  between  stratified  beds  which  infold  them.  If 
in  the  forming  of  an  intrusive  sheet  the  supply  of  material 
from  below  goes  on  faster  than  it  can  spread  at  a  given 
thickness  laterally,  the  strata  above  will  begin  to  arch  up, 
and  if  the  process  continues  a  great  thick  half  lens, 
flat  below  and  rounded  above,  of  liquid  rock,  will  be 
formed.*  A  cross  section  of  such  a  one  is  shown  in  Fig. 
63.  They  are  apt  to  run  out  into  intrusive  sheets  or  be 
accompanied  by  them.  Also  on  the  flanks  of  folding, 
uplifting  mountain  ranges  where  the  folding  strata  are 
subjected  to  horizontal  pressure  they  may  tend  to  open, 
*  Increased  viscosity  of  magma  also  helps  in  this  result. 


PLATE    3. 


LACCOLITH,    AND    INTRUSIVE    SHEETS   OF   BASALT,    IN 

SANDSTONES,  SHONKIN   SAG,    MONTANA. 

(U.  S.  Geological  Survey.) 


GENERAL  PETROLOGY  OF  IGNEOUS  ROCKS      137 


and  such  openings  be  filled  with  magma  from  below  in 

measure   as   they   open,    as   illustrated   in   Fig.    64.     In 

general,  laccoliths  are 

more  or  less  circular  or 

oval  in    ground    plan, 

and   while    sometimes 

symmetrical  as  in  the 

diagrams  they  are  apt 

not  to  be  so  but  more 


Fig.  63.    Cross  Section  of  a  Laccolith 


or  less  distorted  in  shape.  The  floor  may  be  flat  .or  tilted 
as  in  the  figures.  They  differ  from  intrusive  sheets  only 
in  being  extremely  thick  in  comparison  with  their  lateral 

extension,  and  all  gra- 
dations between  the  two 
may  be  found.  They 
may  be  a  mile  or  more 
in  thickness  and  a  num- 
ber of  miles  in  diameter, 
or  but  a  few  hundred 

Fig.  64.    Section  of  an  inclined  Laccolith     yardg    ^^       The    bedg 

above  are  usually  stretched,  thinned  and  broken  in  the 
process  of  formation.  Like  intrusive  sheets  they  alter 
and  change  the  strata  above  and  below  by  contact 
metamorphism.  They  are  most  apt  to  occur  in  weak 
beds  of  shale,  etc.,  the  stronger,  thicker  beds  of  sand- 
stone and  limestone  being  up-arched.  The  best  exam- 
ples that  are  known  are  found  in  the  region  of  the 
Rocky  Mountains,  where  in  many  places  they  are  not 
uncommon. 

Cases  have  been  described  where  the  roof  of  a  laccolith  has  been 
ruptured  and  driven  upward  by  the  magma  rising  like  a  plug  through 
the  strata.  It  has  been  suggested  that  such  forms  be  called  bys- 
maliths  (Greek,  plug  rocks).  It  has  also  been  suggested  that  when 
a  body  of  magma  is  injected  into  the  stratified  rocks  like  a  laccolith, 
but  of  indefinite  shape  and  without  the  relations  to  the  planes  of 
stratification  which  a  laccolith  has,  such  a  mass  be  termed  a  chonolith 
(Greek,  mold  —  used  in  casting —  rock). 


138  ROCKS  AND  ROCK  MINERALS 

Necks.  When  a  volcano  ceases  its  activity  and  becomes 
extinct  the  column  of  magma,  occupying  the  conduit 
leading  to  unknown  depths  below,  may  solidify  and  form 
a  mass  of  igneous  rock.  Erosion  may  cut  away  a  great 
part  of  the  light  porous  ashes  and  lavas,  leaving  this  more 
solid  and  resistant  rock  projecting,  as  shown  by  the 
heavy  line  abc  in  Fig.  65.  Or  the  level  of  erosion  may 
continue  to  descend  into  the  rocks  which  form  the  base- 
ment on  which  the  volcano 
is  built,  all  traces  of  the 
ashes,  lavas,  etc.,  being 
swept  away  and  only  this 
mass  being  left  to  mark 
its  former  site.  Such  a 

Fig.  65.     Section  Through  a  Volcano  ,          .    .     . 

mass  01  rock  is  known  as 

a  volcanic  neck.  It  is  commonly  more  or  less  circular 
in  ground  plan  and  may  be  from  a  few  hundred  yards  up 
to  a  mile  or  more  in  diameter.  The  rocks  about  them 
are  apt  to  be  fissured  and  filled  with  dikes  and  in  many 
cases,  if  stratified,  with  intrusive  sheets. 

Stocks.  This  term  has  been  applied  to  large  bodies  of 
intrusive  rock  which  in  the  form  of  magma  have  ascended 
into  the  upper  region  of  the  earth's  crust  and  there  solid- 
ified. They  have  become  visible  by  extended  erosion  and 
tend  to  have  a  more  or  less  circular  or  elliptic  ground 
plan.  Their  plane  of  contact  cuts  across  the  inclosing 
rocks,  is  more  or  less  irregular,  and  the  mass  may  widen  in 
extent  as  it  descends.  Their  size  may  be  anything  from 
a  few  hundred  yards  to  many  miles  in  extent.  Since 
they  are  apt  to  form  protuberant  topographic  features 
through  erosion  they  are  sometimes,  especially  in  Great 
Britain,  called  bosses.  The  distinction  from  a  volcanic 
neck  is  not  one  of  size  alone,  though  necks  tend  to  be 
smaller  than  stocks,  but  lies  in  the  fact  that  the  term 
neck  is  employed  only  when  there  is  evidence  that  there 
has  been  extrusive  volcanic  activity  from  it.  Some 
stocks  were  doubtless  necks,  but  this  cannot  now  be 


PLATE  4. 


r  "3 


o 
o  o 

<J     02 


GENERAL  PETROLOGY  OF  IGNEOUS  ROCKS     139 

proved.  The  granite  hills  of  New  England,  of  Scotland 
and  of  other  old  eroded  mountain  regions  are  often  stocks 
or  bosses. 

Bathyliths.  This  term  is  used  in  a  general  way  to 
designate  those  huge  irregular  masses  of  igneous  rock, 
which,  underlying  the  sedimentary  and  metamorphic 
ones  or  sometimes  cutting  through  them,  have  been 
exposed  by  erosion.  They  are  seen  in  the  oldest  exposed 
areas  of  the  crust  where  they  are  characteristically  accom- 
panied or  surrounded  by  metamorphic  rocks,  as  in  eastern 
Canada,  or  in  mountainous  regions  where  they  form  the 
central  cores  or  masses  of  the  ranges,  as  in  the  Alps. 
They  differ  chiefly  from  stocks  in  their  much  greater  size, 
as  they  are  not  infrequently  many  thousands  of  square 
miles  in  surface  area. 

While  some  stocks  are  clearly  intrusive  and  have  displaced  the 
rocks  whose  site  they  occupy,  the  mode  of  formation  of  others  and 
of  bathyliths  is  still  a  subject  of  speculation.  Some  have  held  that 
they  have  attained  their  position  by  melting  and  assimilating  the 
previous  formation  and  thus  replacing  it,  while  others  have  urged 
the  view  that  it  has  been  ruptured,  uplifted  and  driven  out  by  the 
invading  mass  and  then  eroded  away.  Various  modifications  of 
these  views  have  been  suggested,  but  geologic  science  is  not  yet  in  a 
position  to  pronounce  definitely  upon  their  correctness. 

Extrusive  Igneous  Rocks.  For  petrographical  purposes 
two  chief  modes  of  extrusion  may  be  recognized,  the 
quiet  one,  giving  rise  to  outwellings  of  magma  in  the  liquid 
state  which  then  solidifies  to  rock,  and  the  explosive,  in 
which  the  material  by  the  violent  action  of  gases  is  pro- 
jected into  the  air  and  falls  in  a  solid  but  fragmental  form. 

Quiet  Eruption;  Lava  Flows.  When  the  magma  rises 
to  the  surfaces  and  outpours  it  is  then  called  lava.  The 
solidified  material  is  often  called  a  sheet  of  lava  or  extrusive 
sheet.  Such  flows  often  come  from  volcanoes;  the 
extrusions  of  some,  like  those  now  active  in  Hawaii,  being 
wholly  of  this  nature,  while  in  others  they  alternate  with 
or  succeed  projections  of  explosive  fragmental  material. 


140 


ROCKS  AND  ROCK  MINERALS 


In  other  cases  they  are  not  connected  with  volcanic 
eruptions  but  have  taken  place  as  quiet  outwellings  from 
numerous  fissures.  This  has  sometimes  occurred  on  a 
huge  scale,  as  in  the  Columbia  River  region  of  the  north- 
western United  States,  in  western  India  and  in  the  north 
of  the  British  Isles.  In  these  regions  the  repeated  lava 
flows  are  thousands  of  feet  in  depth  and  cover  areas  of 
from  100,000  to  as  much  as  200,000  square  miles. 

Not  infrequently  sheets  of  lava  have  sunk  below  sea-level  and 
been  covered  by  deposits,  or  they  have  originated  on  the  sea  floor  and 
have  been  covered.  Such  buried  extrusive  sheets  are  distinguished 
from  intrusive  ones  by  the  fact  that  they  have  not  altered  or  changed 
the  sediments  above  them  by  contact  metamorphism  (qu.  vid.),  and 
their  upper  surfaces  usually  show  the  structures  common  to  the 
surface  of  lavas,  such  as  the  vesicular,  amygdaloidal,  scoriaceous 
and  ropy  ones  described  later. 

Explosive  Eruption;  Tuffs  and  Breccias.  When  a 
magma  attains  the  surface  in  the  canal  of  a  volcano  it 
may  give  rise  to  quiet  flows  of  lavas  as  mentioned  above, 
or  if  its  viscosity  is  sufficient  and  it  is  charged  with  vapors 
under  great  tension  it  will  give  rise  to  explosive  activity, 


Fig.  66.  Diagram  to  Illustrate  the  Occurrence  of  Igneous  Rocks  :  6,  bathylith; 
s,  stock;  n,  volcanic  neck  forming  v,  a.  volcano  with  tuffs  and  breccias;  /,  I,  lacco- 
liths; i,  intrusive  sheet;  6,  extrusive  sheet;  d,d,  dikes.  Horizontal  distance 
shown,  thirty  miles;  vertical  distance,  three  miles. 

and  the  material  will  be  projected  into  the  air  to  fall  in 
solid  fragmental  form.  Owing  to  the  expansion  of  the 
vapors,  chiefly  steam,  the  projected  pieces  usually  have 
a  more  or  less  pronounced  vesicular  structure,  and  vary 
in  size  from  those  weighing  perhaps  several  hundred 


GENERAL  PETROLOGY  OF  IGNEOUS  ROCKS      141 

pounds  to  dust  so  fine  that  it  floats  for  long  periods  in  the 
air.  According  to  size  these  may  be  roughly  classified 
as  follows.  Pieces  the  size  of  an  apple  and  upward  are 
called  volcanic  bombs;  those  the  size  of  nuts  are  termed 
lapilli;  those  the  size  of  small  peas  or  shot  volcanic  ashes; 
while  the  finest  is  volcanic  dust.  The  coarser  material, 
the  bombs,  ashes  and  lapilli,  falls  around  the  vent  and 
builds  up  the  cone;  the  lighter  ashes  and  dust,  carried 
by  air  currents,  tend  to  fall  after  these  and  at  greater 
distances.  The  beds  of  coarser  material  thus  produced 
are  termed  volcanic  conglomerate  or  more  commonly  vol- 
canic breccia,  while  the  finer  is  known  as  tuff.* 

General  Characters  of  Igneous  Rocks. 

Since  igneous  rocks  are  formed  by  the  consolidation  of 
molten  magmas  it  is  evident  that  the  nature  of  a  rock 
produced  must  in  large  measure  depend  upon  the  chem- 
ical composition  of  the  magma  which  forms  it.  For 
most  rocks  are  composed  of  mineral  grains,  and  the  kinds 
and  relative  amounts  of  these  must  depend  upon  the 
kinds  and  relative  amounts  of  the  chemical  elements 
which  form  the  molten  fluid.  It  is  pertinent  therefore 
to  inquire  what  the  general  chemical  character  of  the 
earth's  magmas  is  like  and  if  there  are  any  general  rules 
which  appear  to  govern  their  composition. 

Chemical  Composition  of  Magmas.  We  cannot  of 
course  subject  a  molten  magma  directly  to  investigation, 
but  this  may  be  essentially  done  if  an  average  sample  of 
an  igneous  rock  is  subjected  to  chemical  analysis.  Several 
thousand  such  analyses  have  been  made  of  rocks  from 
all  parts  of  the  world,  and  these  results  show,  as  might  be 
expected  from  the  discussion  given  on  page  17  and 
following,  that  the  magmas  and  therefore  the  rocks  are 

*  (Volcanic  tuff  was  formerly  commonly  called  volcanic  tufa,  but 
at  the  present  time  it  is  customary  to  restrict  the  word  tufa  to 
deposits  from  aqueous  solution,  especially  those  of  a  calcareous 
nature.) 


142 


ROCKS  AND  ROCK  MINERALS 


made  up  of  the  following  oxides:  silica,  SiO2;  alumina, 
A1203;  iron  oxides,  both  ferric,  Fe2O3,  and  ferrous,  FeO; 
magnesia,  MgO;  lime,  CaO;  soda,  Na2O,  and  potash,  K2O. 
Other  oxides,  including  water,  are  also  present  but  in 
such  relatively  small  amounts  that  they  do  not  exercise 
any  controlling  influence  and  may  be  neglected. 

The  variations  in  chemical  composition  which  are 
shown  in  the  magmas  are  in  a  general  way  exhibited  in 
the  following  table  of  selected  analyses. 


I. 

II. 

III. 

IV. 

V. 

VI. 

VII. 

VIII. 

SiO,  . 
ALA 

56.6 
22.4 

65.5 
17.8 

72.5 
13.1 

65.1 
16.2 

56.0 
15.6 

49.2 
12.0 

40.1 
7.8 

38.4 
0.3 

Fe203 

1.8 

0.7 

1.7 

1.1 

1.2 

2.8 

7.3 

3.4 

FeO  . 

0.8 

1.2 

1.0 

3.2 

6.3 

8.8 

8.6 

6.7 

MgO  . 

1.3 

1.0 

0.6 

2.3 

6.8 

9.3 

23.7 

45.2 

CaO  . 

0.3 

1.9 

1.0 

4.0 

7.3 

10.6 

6.5 

0.4 

Na2O. 
K20  . 

8.5 
7.3 

5.6 
5.6 

4.2 
4.9 

4.0 
2.5 

2.2 
1.3 

1.9 
1.7 

1.2 
0.5 

jo.: 

Rest  . 

1.4 

0.7 

0.7 

1.6 

3.3 

3.2 

4.5 

5.7 

Total     . 

100.4 

100.0 

99.7 

100.0 

100.0 

99.5 

100.2 

100.2 

I,  Nephelite  Syenite,  Serra  di  Monchique,  Portugal;  II,  Syenite, 
Highwood  Mountains,  Montana;  III,  Granite,  Castle  Mountains, 
Montana;  IV,  Quartz  Diorite,  Electric  Peak,  Yellowstone  Park; 
V,  Diorite,  Montgomery  County,  Maryland;  VI,  Gabbro,  Red 
Mountains,  Montana;  VII,  Peridotite,  Devonshire,  England;  VIII, 
Dunite,  Tulameen  River,  British  Columbia. 

Variation  of  Magmas  and  Mineral  Composition.     It  is 

not  to  be  understood  that  all  the  different  varieties  of 
magmas  are  represented  by  these  analyses;  they  are  only 
selected  to  show  the  most  prominent  and  general  features 
of  variation.  Certain  of  these  can  be  readily  seen  by 
observing  the  table.  Thus  in  the  first  three  analyses  it 
is  evident  that  silica,  alumina  and  the  alkalies,  potash 
and  soda,  are  the  chief  oxides  composing  them,  while 
lime,  iron  and  magnesia  play  a  very  subordinate  part. 


GENERAL  PETROLOGY  OF  IGNEOUS  ROCKS     143 

It  is  therefore  evident  that  if  such  magmas  should 
crystallize  into  minerals  they  would  be  mostly  composed 
of  alkalic  feldspars  because  these  are  composed  of  silica, 
alumina  and  alkalies.  Again,  if  we  regard  the  amounts 
of  silica  in  these  three  and  remember  that  orthoclase, 
potash  feldspar,  contains  about  65  per  cent  of  silica  and 
albite,  the  soda  feldspar,  about  68,  it  is  clear  that  in 
No.  Ill  there  is  more  silica  than  needed  to  form  the  alka- 
lies and  alumina  into  feldspars,  and  there  will  therefore 
be  extra  silica  which  will  crystallize  as  free  quartz.  In 
No.  I,  on  the  contrary,  there  is  not  enough  silica  to  turn 
all  of  the  alumina  and  alkalies  into  feldspar,  and  a  certain 
amount  of  some  mineral,  such  as  nephelite,  which  con- 
tains these  oxides  in  combination  with  a  smaller  amount 
of  silica  must  be  formed  to  compensate  this  deficiency. 
In  No.  II  the  per  cent  of  silica  is  very  nearly  that  required 
for  the  pure  feldspars,  and  these  will  make  up  the  great 
bulk  of  the  rock  with  little  either  of  quartz  on  the  one 
hand,  or  of  nephelite  on  the  other. 

If  now  we  turn  our  attention  to  the  oxides  of  lime, 
iron  and  magnesia,  it  is  evident  that  the  minerals  which 
they  produce,  such  as  biotite,  hornblende  and  pyroxene, 
will  have  but  a  subordinate  role  in  the  first  three  rocks, 
but  in  Nos.  IV-IX  these  oxides  continually  increase 
while  silica  alumina  and  alkalies  also  decrease,  and  even- 
tually the  last  two  vanish  and  the  silica  becomes  very 
low.  Expressing  this  in  terms  of  minerals,  if  the  magmas 
crystallized,  it  is  evident  that  in  these  four  the  ferro- 
magnesian  minerals  —  those  containing  iron  or  magnesia 
or  more  commonly  both  —  such  as  pyroxene,  amphibole 
or  olivine,  would  play  an  increasingly  important  role,  and 
that  the  last  rock  would  be  wholly  composed  of  them, 
while  feldspars  correspondingly  become  less  important 
and  ultimately  disappear. 

In  this  connection  the  variation  of  lime  deserves  a  sepa- 
rate word  because  lime  has  a  dual  function :  it  may  form 
a  feldspar  with  alumina  and  silica  which  then  commonly 


144 


ROCKS  AND  ROCK  MINERALS 


combines  with  soda  feldspar  to  form  plagioclase  (soda- 
lime  feldspar),  or  it  may  enter  into  the  ferromagnesian 
minerals,  pyroxene  and  amphibole.  It  generally  does 
both  and  thus  for  a  time  as  we  follow  the  table  of  analyses 
from  left  to  right,  as  lime  increases,  the  quantity  of  both 
plagioclase  and  of  ferromagnesian  minerals  increases  also. 
Coincident  with  this  the  alumina  also  increases  somewhat. 
Variation  shown  by  Diagrams.  The  facts  which  have 
been  stated  above  may  be  shown  in  a  graphic  manner  by 

means  of  a  simple  dia- 
gram, Fig.  67.  Thus  in 
the  place  of  the  analyses 
of  the  foregoing  table 
we  may  draw  verti- 
cal lines,  one  for  each 
analysis,  at  equal  dis- 
tances apart  and  on  each 
line  set  off  a  vertical 
distance  in  millimeters 
equal  to  the  per  cents 
of  each  oxide  in  the 
analysis.  Through  these 
points  lines  are  drawn 
corresponding  to  each 
oxide,  the  iron  and 
magnesia  from  the  simi- 
larity of  function  they 

exhibit  being  united  in  one  line.  The  equal  distances  for 
each  analysis  at  the  foot  of  the  diagram  thus  serve  as 
abscissas  and  the  percentages  are  ordinates,  while  the 
connecting  lines  approach  curves  which  show  the  mutual 
relations  of  the  oxides.  In  the  description  of  the  variation 
of  the  oxides  it  was  pointed  out  how  this  caused  a  corre- 
sponding variation  in  the  minerals  produced  by  the  crys- 
tallization of  the  magmas  composed  of  these  oxides.  By 
considering  the  relative  amounts  of  the  important  minerals 
which  each  type  of  analysis  would  produce  we  can  con- 


Fig.  67.    Diagram  to  Illustrate  Chemical 
Variation  of  Igneous  Rocks 


PLATE    5. 


B.    Syenite,  mostly  Feldspar. 


C.    Diorite,  some  Feldspar. 


D.    Peridotite,  no  Feldspar. 


CONTRAST  OF   FELDSPATHIC  AND   FERROMAGNESIAN 
ROCKS. 


GENERAL  PETROLOGY  OF  IGNEOUS  ROCKS      145 

struct  a  diagram,  Fig.  68,  which  will  show  the  variation 
of  the  minerals  in  a  general  way  in  the  common  rocks. 
It  also  shows  the  relative  proportions  of  the  minerals  in 
the  more  common  and  important  kinds  of  igneous  rocks, 


Fig.  68.    Diagram  to  Illustrate  the  Variations  and  Relative  Proportions  of  the 
Minerals  Composing  the  Important  Igneous  Rocks. 

and  it  serves  as  a  basis  for  their  classification  as  will  be 
explained  later.  The  relative  proportions  of  the  minerals 
are  given  in  the  vertical  direction,  the  variation  and  pas- 
sage of  one  kind  of  rock  into  another  in  the  horizontal 
direction. 

It  should  be  repeated  that  these  diagrams  and  the 
table  of  analyses  are  not  to  be  taken  in  a  hard  and  fast 
manner  as  representing  the  limits  of  variation  and  all  the 
possible  mineral  combinations  of  igneous  rocks.  This 
would  be  very  wide  of  the  truth.  Other  analyses  might 
be  selected  which  would  yield  different  diagrams,  and  if 
of  rare  and  uncommon  rocks,  they  might  be  very  different 
indeed,  but  in  a  general  way  these  may  be  accepted  as 
showing  the  more  important  chemical  and  mineralogical 
features  which  distinguish  the  common  kinds  of  igneous 
rocks  from  one  another. 

Minerals    of    Igneous    Rocks.     From    what    has    been 


146  ROCKS  AND  ROCK  MINERALS 

stated  in  the  foregoing  sections  it  is  evident  that  the  more 
important  minerals  which  compose  the  igneous  rocks  are 
the  feldspars,  quartz  and  the  ferromagnesian  group.  For 
purposes  of  classification  to  be  explained  later  it  is  con- 
venient to  contrast  the  ferromagnesian  on  the  one  hand 
with  the  quartz  and  feldspars  on  the  other.  Recalling 
that  silica  (Si02)  and  alumina  (A/203)  are  prominent 
substances  in  the  composition  of  these  latter  minerals, 
and  following  American  petrographic  usage  we  may  term 
this  group  the  salic  one.  More  specifically  the  prominent 
minerals  of  the  igneous  rocks  are  given  in  the  following 
table: 

SALIC  GROUP.  FERROMAGNESIAN  GROUP. 
Alkalic  Feldspar  Pyroxenes 
Plagioclase  Feldspar                      Amphiboles 
Quartz                                             Biotite 
Olivine 


Nephelite 

Sodalite  Iron  Ores 

Corundum 


The  last  three  in  the  salic  group  are  of  much  less  impor- 
tance than  the  first  three  on  account  of  their  restricted 
occurrence;  the  iron  ores,  hematite,  ilmenite  and  mag- 
netite, though  so  widely  distributed  that  nearly  all  igneous 
rocks  contain  one  or  more  of-  them,  are  of  less  importance 
than  the  other  ferromagnesian  minerals  because  they 
usually  form  only  a  very  small  proportion  of  all  the 
minerals  in  the  rocks.  A  mineral,  like  these,  which  may 
be  quite  evenly  distributed  through  a  rock  but  makes  only 
a  small  part  of  its  mass  is  called  an  accessory  component 
in  contradistinction  to  those  which  form  its  main  bulk 
and  are  termed  chief  or  essential  components. 

The  chemical  and  physical  characters  of  the  minerals 
mentioned  in  the  above  list  have  been  described  under 
their  appropriate  headings  in  Part  II,  to  which  reference 
may  be  made,  for  further  information  concerning  them. 

Order  of  Crystallization.     If  a  polished  surface  of  a 


GENERAL  PETROLOGY   OF  IGNEOUS  ROCKS     147 


coarse-grained  rock  be  attentively  studied  with  a  lens,  or 
better  if  a  thin  section  be  observed  under  the  microscope, 
it  will  usually  be  found  that  there  are  more  or  less  distinct 
evidences  that  all  the  minerals  composing  it  have  not 
crystallized  simultaneously  but  successively.  Thus  in 
Fig.  69  the  crystals  of  biotite  mica  (M)  contain  grains  and 
octahedrons  of  black  iron  ore,  magnetite;  they  occur  also 
in  the  other  minerals.  They  are  evidently  older  than  the 
mica  because  they  are  inclosed  by  it.  The  mica  is  older 
than  the  soda-lime  feldspars  or  plagioclases  (P)  because 
it  abuts  into  them  with  its  own  crystal  faces  or  is  partly 


Fig.  69.    Diagram  to  Illustrate  Successive  Crystallization 

inclosed  by  them  as  they  grow  around  the  already  formed 
crystals.  In  the  same  way  the  plagioclase  has  its  own 
form  as  regards  the  alkali  feldspar,  orthoclase  (O)  and 
the  quartz  (Q),  which  surround  it,  and  is  therefore 
judged  to  be  older  than  they  are.  When  the  orthoclase 
and  quartz  are  considered  they  do  not  show  any  crystal 
boundaries  with  respect  to  one  another,  and  their  crys- 
tallization is  therefore  judged  to  be  more  nearly  simul- 


148  ROCKS  AND  ROCK  MINERALS 

taneous.  The  order  of  crystallization  as  thus  worked  out 
in  this  particular  case  is:  first,  magnetite,  then  biotite 
mica,  then  plagioclase,  and  lastly  orthoclase  and  quartz. 

The  studies  which  have  been  made  of  igneous  rocks 
teach  that  in  general  the  order  of  crystallization  is :  first, 
the  oxides  or  ores  of  iron,  then  ferromagnesian  minerals, 
then  soda-lime  feldspars,  then  alkalic  feldspars  (and 
feldspathoids)  and  lastly  quartz.  One  observes  from 

this,    as    illustrated    in    the 

1.  Magnetite,  Fe304.  adjoining     table      that     the 

2.  Pyroxene,  (MgFe)Ca(SiO3)2.    process  begins  with  metallic 

Q   P!o.r™ia  *  ( mCaAl2Si208-       oxides     which     contain     no 

3.  Plagioclase,  I    ,,    .,J.  i  8  ...  , 

UNaAlSi3O8.         silica,    that    next   come   the 

4.  Orthoclase,  KAlSi3O8.  ferromagnesian      minerals, 

5.  Quartz.  SiO2.  ,,  ,  .,. 

ortho  and  metasilicates,  then 

feldspars  which  contain  more 

silica  and  finally  quartz  or  free  silica.  Thus  there  tends 
to  crystallize  out  successively  minerals  richer  and  richer  in 
silica.  It  is  not  to  be  understood  however  that  one  mineral 
necessarily  finishes  its  period  of  crystallization  before 
another  one  begins  as  in  A,  but  rather  that  they  overlap 


Flag.  Orthoclase.  Quartz. 


urtnoci! 

Quartz. 


as  in  B,  that  is,  that  one  may  begin  before  another  has 
finished,  and  continue  after  the  former  has  ceased.  Expe- 
rience shows  that  with  orthoclase  and  quartz  the  overlap 
is  so  great  that  they  crystallize  nearly  simultaneously, 
only  orthoclase  usually  begins  and  quartz  finishes. 

Insolubility  vs.  Infusibility.  A  molten  silicate  magma  is 
to  be  regarded  as  a  complex  solution  of  some  compounds  in 
others,  like  a  solution  of  mixed  salts  in  some  solvent  such 


GENERAL  PETROLOGY  OF  IGNEOUS  ROCKS      149 

as  water.  As  the  heated  solution  cools,  a  point  is  reached 
where  some  compound,  or  mineral,  becomes  insoluble  in 
the  resulting  solution  and  it  therefore  crystallizes  out. 
The  statement  that  is  sometimes  made  that  the  minerals 
crystallize  in  the  order  of  their  fusibility  is  entirely  wrong; 
thus  from  the  preceding  paragraph  we  see  that  pyroxene 
crystallizes  before  quartz;  now  pyroxene  is  rather  readily 
fusible  before  the  blowpipe,  while  quartz  is  infusible.  It 
is  not  therefore  a  question  of  infusibility  but  of  solubility 
which  determines  the  order  of  crystallization. 

Influence  of  Mineralizers.  Experience  teaches  us  that 
those  magmas  which  attain  the  surface  in  volcanoes  and 
in  lava  flows  contain  large  quantities  of  volatile  sub- 
stances, especially  water  vapor,  which  they  give  off, 
frequently  with  explosive  violence.  It  was  formerly 
considered  that  the  magmas  imbibed  these  from  the 
moisture  laden  rocks  with  which  they  came  in  contact  on 
their  way  to  the  surface.  At  present  these  volatile  sub- 
stances are  generally  held  to  be  wholly  or  in  large  part  of 
magmatic  origin,  that  is,  original  constituents  of  the 
earth's  interior  molten  masses,  contained  therein  under 
pressure.  Without  further  regard  to  the  theories  of  how 
they  came  to  be  there  we  know  that  the  magmas  contain 
them  and  that  they  are  of  great  importance  in  a  number 
of  ways  in  the  formation  of  igneous  rocks.  The  most 
important  of  these  is  water,  but  carbon  dioxide,  fluorine, 
boric  acid,  sulphur  and  chlorine  are  also  prominent  and 
may  produce  important  results.  The  work  of  various 
investigators,  especially  the  French,  has  shown,  that 
while  some  minerals  such  as  pyroxene,  magnetite,  lime 
feldspar,  olivine  and  nephelite  may  be  artificially  pro- 
duced by  fusing  their  constituents  together  and  allowing 
the  molten  mass  to  cool  slowly,  other  minerals  such  as 
hornblende,  biotite,  orthoclase  and  quartz  do  not  form 
in  dry  fusions  in  the  same  way.  For  their  production 
more  or  less  of  the  volatile  substances  mentioned  above 
must  be  present,  especially  water  vapor.  These  sub- 


150  ROCKS  AND  ROCK  MINERALS 

stances  appear  to  act  in  two  ways:  in  one  in  a  chemical 
manner  since  some  minerals,  such  as  biotite  and  horn- 
blende, always  contain  small  quantities  of  water  (in  the 
form  of  hydroxyl,  -  OH)  or  fluorine  or  both,  and  these 
are  consequently  necessary  for  their  production;  and 
second,  in  a  physical  manner  in  that  they  lower  the 
melting  point  of  the  fusion  and  greatly  increase  its  fluidity. 
Thus  orthoclase,  albite  and  quartz  which  have  extremely 
high  melting  points  but  only  crystallize  at  much  lower 
temperatures,  in  a  dry  fusion  become  so  viscous  on  cooling 
that  they  are  unable  to  crystallize  and  therefore  solidify 
as  glasses.  The  addition  of  water  under  pressure  lowers 
the  temperature  of  solidification  and  increases  the  fluidity 
or  mobility  of  the  melted  mass  and  permits  such  move- 
ment of  the  molecules  that  they  can  arrange  themselves 
in  crystal  form,  and  the  above  minerals  are  produced. 
These  substances  then,  such  as  water,  fluorine,  etc.,  which 
exert  so  important  a  function  in  processes  of  crystalliza- 
tion and  on  the  formation  of  igneous  rocks  are  called 
mineralizers.  As  crystallization  progresses,  the  amount 
of  them,  beyond  what  is  chemically  (and  to  some  extent 
mechanically)  retained  in  the  minerals,  is  gradually 
excluded  from  the  solidifying  rock  mass  to  play  an  active 
role  in  new  and  important  processes,  such  as  the  forma- 
tion of  pegmatites,  contact  metamorphism  and  others 
which  will  be  described  later  in  their  appropriate  places. 

Texture  of  Igneous  Rocks. 

It  has  been  pointed  out  that  igneous  rocks  vary  in  the 
kinds  and  proportions  of  the  minerals  that  compose  them 
and  that  this  variance  is  mainly  due  to  the  chemical 
composition  of  the  magmas  from  which  they  are  derived. 
Another  important  way  in  which  these  rocks  vary  is  in 
their  texture.  Thus  one  rock  may  be  made  up  of  mineral 
grains  so  large  that  the  different  minerals  are  easily 
distinguished,  while  in  another  the  grains  are  so  small  as 
to  defy  identification  by  the  eye  or  simple  lens.  Again 


GENERAL  PETROLOGY  OF  IGNEOUS  ROCKS     151 

the  grains  may  be  approximately  all  of  about  one  size  or 
they  may  vary  in  size,  some  being  relatively  large  while 
the  rest  are  minute.  Or  again  the  conditions  may  have 
been  such  that  the  magma  had  no  opportunity  to  crys- 
tallize but  solidified  as  a  simple  glass,  or  to  only  partly 
crystallize  and  formed  a  mixture  of  glass  and  crystals. 
Such  variations  for  the  most  part  are  independent  of 
chemical  composition,  they  depend  upon  the  physical 
conditions  under  which  the  molten  mass  has  solidified,  and 
thus  a  magma  of  a  given  composition  may  appear  in  any 
one  of  the  states  mentioned  above  if  subjected  to  the 
proper  conditions. 

The  characteristic  features  which  a  rock  exhibits  in  this 
respect  constitute  its  texture,  and  rocks  are  distinguished 
and  classified  in  one  way  according  to  their  textures, 
just  as  in  another  way  they  are  distinguished  and  classified 
according  to  their  mineral  composition. 

Factors  influencing  Texture.  If  a  strong  hot  solution 
of  a  salt,  such  as  common  alum  in  water,  is  allowed  to 
cool  very  slowly  and  regularly,  comparatively  few  centers  of 
crystallization  will  be  set  up,  and  the  few  resulting  crystals 
will  have  a  long  period  of  growth  and  will  be  of  good  size. 
If,  on  the  contrary,  the  cooling  is  very  rapid  a  great  number 
of  centers  of  crystallization  will  form,  the  period  of  growth 
will  be  short  and  a  great  number  of  very  small  crystals 
will  result.  The  same  is  true  in  the  molten  liquids  from 
which  the  igneous  rocks  are  formed.  If  the  cooling  has 
progressed  with  great  slowness  and  regularity  then 
coarse-grained  rocks  are  produced;  if  the  cooling  is  rapid 
then  they  are  fine-grained  and  the  cooling  may  take  place 
so  quickly  that  there  is  no  opportunity  for  complete 
crystallization,  and  rocks  wholly  or  in  part  composed  of 
glass  will  result.  The  rate  of  cooling  then  is  a  prominent, 
and  in  fact  the  most  prominent,  factor  in  the  production 
of  rock  texture.  In  addition  to  the  temperature  there 
has  been  a  tendency  in  the  past  to  ascribe  also  a  prominent 
role  to  the  pressure.  The  idea  involved  is  that  if  a  magma 


152  ROCKS  AND  ROCK  MINERALS 

remained  liquid  within  the  earth  at  a  given  temperature 
and  if  for  any  reason  the  pressure  increases,  a  point  will 
be  eventually  reached  where  it  will  be  forced  to  crystallize 
and  become  solid,  since  in  so  doing  its  volume  would 
be  reduced.  Decrease  of  temperature  and  increase  of 
pressure  would  then  work  together.  While  this  may  be 
true  in  theory  it  does  not  seem  probable  that  the  pressures 
obtaining  in  the  upper  region  of  the  crust  are  a  very 
prominent  factor  in  this  direction,  since  geological  obser- 
vation has  shown  that  a  particular  variety  of  texture  can 
be  found  unchanged  through  a  range  of  10,000  feet 
vertical.  Still  it  cannot  be  denied  that  pressure  probably 
has  some  influence  on  the  process  of  crystallization  and 
the  production  of  rock  texture. 

The  presence  of  mineralizers,  especially  water,  has 
undoubtedly  a  strong  influence  on  the  texture,  particu- 
larly in  the  siliceous  rocks,  for  this  greatly  increases  the 
fluidity  of  such  magmas  and,  as  they  cool  down  and  the 
crystallizing  points  of  the  different  minerals  are  reached, 
they  still  retain  their  mobility  instead  of  becoming  stiffly 
viscous.  This  increases  the  range  of  movement  of  the 
mineral  molecules  forming,  and  enables  larger  crystals  to 
grow  and  a  coarser  texture  to  be  produced.  As  we  shall 
see  later  this  reaches  its  maximum  in  the  pegmatite  dikes. 

In  connection  with  what  has  just  been  stated  chemical 
composition  of  the  magmas  has  a  certain  influence  in 
producing  texture.  This  shows  itself  in  two  ways.  Those 
magmas  which  are  deficient  in  silica  and  especially  those 
which  contain  much  iron  and  magnesia  and  which  are 
shown  in  the  right  hand  side  of  the  diagrams  given  on  a 
previous  page  remain  liquid,  without  becoming  stiffly 
viscous,  to  much  lower  temperatures  than  those  with 
high  silica  which  are  found  expressed  in  the  middle  of  the 
diagrams.  This  liquid  condition  enables  them  to  crys- 
tallize more  freely  and  to  form  in  consequence  coarse- 
textured  rocks  under  circumstances  where  the  siliceous 
magmas  would  produce  only  types  fine-grained  in  texture 


GENERAL  PETROLOGY  OF  IGNEOUS  ROCKS     153 

or  even  glassy  from  inability  to  crystallize  completely, 
owing  to  increasing  viscosity.  The  other  way  in  which 
chemical  composition  influences  texture  is  this.  Dif- 
ferences in  composition  in  the  magmas  mean  of  course 
differences  in  the  kinds  of  minerals  which  they  produce. 
Different  minerals  crystallize  in  different  shapes  and 
although,  owing  to  interference  with  one  another,  they 
may  not  form  in  perfect  crystals,  they  tend  to  take  such 
shapes.  Some  form  tabular  shapes,  others  spherical  or 
cuboidal  grains  or  elongated  prisms.  Thus,  while  the 
general  size  of  such  grains  may  remain  the  same  through- 
out a  mass  of  rock,  such  differences  in  shape  will 
produce  corresponding  differences  in  what  we  may  call 
the  pattern  or  fabric  of  the  rock  and  thus  influence  its 
texture. 

Relation  of  Texture  to  Geologic  Mode  of  Occurrence.  It 
is  evident  that  the  condition  most  favorable  for  the  pro- 
duction of  coarse-textured  rocks  —  conditions  described 
in  the  preceding  discussion  as  slow  cooling,  pressure  and 
the  presence  of  mineralizers  —  will,  in  general,  be  best 
realized  when  the  magma  is  in  large  mass  and  deeply 
buried  in  the  earth's  crust  so  that  it  is  completely 
enveloped  by  surrounding  rock  masses.  The  heavy 
cover  retains  the  heat  and  the  mineralizers  and  gives  in 
part  the  pressure.  Such  igneous  rocks,  formed  in  depth, 
will  only  become  exposed  to  our  observation  when  con- 
tinued erosion  has  carried  away  the  superincumbent 
material.  They  are  often  therefore  called  plutonic  or 
abyssal  and  sometimes  massive  rocks,  and  referring  to 
what  has  been  described  as  the  modes  of  occurrence  of 
the  igneous  rocks  it  can  be  seen  that  bathyliths,  stocks 
and  the  lower  part  of  volcanic  necks  may  be  particularly 
expected  to  exhibit  such  texture  and  nearly  always 
do  so. 

On  the  other  hand,  when  the  magmas  attain  the  surface 
and  are  forced  out  in  volcanic  eruptions,  lava  flows,  etc., 
entirely  different  conditions  will  prevail;  there  is  no  cover 


154        ROCKS  AND  ROCK  MINERALS 

to  retain  the  heat  and  the  cooling  in  consequence  is  rapid, 
Also  the  pressure  has  been  relieved,  and  with  loss  of  cover 
and  pressure  the  mineralizers  quickly  depart.  As  a 
result  fine-grained,  dense,  compact  textures  are  formed, 
or  the  cooling  may  be  so  rapid  that  crystallization  may 
fail  to  occur,  either  wholly  or  in  part,  and  rocks  entirely 
or  partly  composed  of  glass  may  be  produced.  When 
rocks  are  more  or  less  glassy  it  is  in  general  very  good 
evidence  that  they  solidified  as  surface  lavas. 

In  the  smaller  intrusive  bodies,  such  as  the  dikes, 
sheets  and  laccoliths,  the  conditions  in  general  are  between 
the  two  sets  just  described.  The  volume  relative  to  the 
surrounding  rocks  is  less,  the  loss  of  heat  and  mineralizers 
more  rapid  than  in  the  stocks  and  bathyliths,  and,  since 
in  general  the  depth  is  less,  the  pressure  is  diminished. 
Thus  the  textures  are  usually  between  those  of  the  larger 
abyssal  masses  and  the  effusive  lavas.  But  the  conditions 
in  these  occurrences  are  apt  to  be  very  variable,  and  in 
accord  with  this  we  find  the  textures  sometimes  dense 
like  the  effusives  —  but  very  rarely  glassy  —  and  some- 
times coarse-granular  like  the  larger  abyssal  masses.  In 
them,  too,  the  function  of  chemical  composition  described 
in  the  preceding  section  is  often  most  strongly  displayed. 
Thus  highly  siliceous  dikes  and  sheets  of  fine  grain  will 
be  found  associated  under  the  same  geological  conditions 
with  other  ones  low  in  silica  and  high  in  iron  and  magnesia 
of  relatively  much  coarser  grain. 

It  is  especially  in  these  occurrences  and  in  the  surface 
lavas  that  the  porphyritic  texture,  to  be  presently  described, 
is  most  liable  to  be  found. 

Textures  of  Igneous  Rocks.  Based  on  the  principles 
which  have  been  enunciated  in  the  foregoing  sections  the 
textures  of  igneous  rocks  for  megascopic  study  may  be 
classified  as  follows: 

Grained.  All  sizes  of  grain  large  enough  to  be  seen 
with  the  unaided  eye.  Example,  ordinary  granite. 


PLATE   6. 


3.  Coarse  Grain. 
EVEN-GRANULAR   TEXTURE. 


GENERAL  PETROLOGY  OF  IGNEOUS  ROCKS     155 

Dense  (aphanitic).  The  rock  is  crystalline,  i.e.  not 
glassy,  but  the  grains  are  too  fine  to  be  perceived  by 
the  eye.  Example,  many  felsites. 

Glassy.  The  rock  can  be  distinctly  seen  to  be  wholly 
or  in  part  composed  of  glass,  as  in  obsidian. 

The  distinctions  stated  above  relate  in  part  to  its 
crystallinity  or  degree  of  crystallization,  for  all  grades  of 
transition  between  rocks  composed  wholly  of  glass,  partly 
of  glass  and  partly  of  crystals  and  wholly  of  crystals 
exist,  though  to  be  perceived  by  the  unaided  eye  the  glass 
must  form  a  great  or  the  greater  part  of  the  rock.  It 
relates  also  in  part  to  the  absolute  sizes  of  the  crystal 
grains  or  what  we  may  term  the  granularity. 

The  phanerocrystalline  (Greek,  <£avepos,  visible)  rocks 
according  to  the  size  of  grain  can  be  divided  as  follows: 
(See  Figs.  1,  2  and  3,  Plate  6.) 

Fine-grained,  the  average  size  of  the  particles  less  than 
1  millimeter  or  as  fine  as  fine  shot. 

Medium-grained,  between  1  and  5  millimeters. 

Coarse-grained,  greater  than  5  millimeters  or  as  great  as 
or  greater  than  peas. 

But  another  very  important  feature  of  texture  is  that 
of  the  pattern  or  fabric  and  this,  for  megascopic  work,  is 
chiefly  due  to  the  relative  sizes  of  the  crystal  grains  in  a 
given  rock.  There  are  two  chief  kinds  of  fabric  which 
may  be  distinguished: 

Even-granular  fabric  (or  texture),  grains  of  approxi- 
mately the  same  general  size. 

Porphyritic  fabric  (or  texture),  grains  of  a  larger  size 
contrasted  with  finer  ones  or  with  glass. 

Even-granular  Texture.  While  this  means  that  in  a 
given  rock  the  crystal  grains  have  approximately  the 
same  general  size,  as  may  be  seen  by  referring  to  Plate  6, 
it  does  not  mean  that  they  have  necessarily  the  same 
shape.  Careful  examination  of  granites  which  have  this 
texture  will  show  that  the  dark  mica  is  in  many  cases 


156  ROCKS  AND  ROCK  MINERALS 

present  in  well  formed  hexagonal  tablets  or  crystals, 
while  the  feldspars  and  quartz  are  in  shapeless  masses, 
or  the  feldspar  tends  to  have  rough  tabular  or  brick-like 
shapes.  This  depends  on  the  order  of  crystallization  as 
previously  explained. 

Porphyritic  Texture.  Porphyry.  In  this  texture,  when 
typically  developed,  there  is  a  sharp  contrast  between 
larger  crystals  with  definite  crystallographic  bounding 
faces,  which  are  termed  phenocrysts  (Greek,  faivuv,  to 
show),  and  the  material  in  which  they  lie  embedded, 
called  the  groundmass.  This  groundmass  may  have  the 
textural  characters  described  on  a  preceding  page,  it 
may  be  even-granular,  coarse  or  fine,  it  may  be  dense 
or  wholly  or  partly  glassy.  A  rock  with  this  textural 
fabric  is  called  a  porphyry.*  Examples  are  shown  in 
Plate  7. 

Great  variations  are  seen  in  the  phenocrysts ;  they  may 
be  extremely  numerous  and  the  amount  of  groundmass 
small  or  the  reverse;  they  may  be  an  inch  or  more  in 
diameter  or  they  may  be  so  small  as  to  require  close 
observation  to  detect  them;  they  may  be  of  light-colored 
feldspars  and  quartz  or  dark-colored  ferromagnesian 
minerals,  hornblende,  augite  and  pyroxene,  or  of  both 
kinds  of  minerals.  Again,  they  may  be  extremely  well 
crystallized  and  afford  such  striking  specimens  of  perfect 
crystal  development  that  they  find  a  place  in  mineral 
cabinets  or  they  may  be  very  poorly  defined  in  crystal 
form.  And  with  increase  in  numbers  and  poor  crystal 
form,  all  degrees  of  transition  into  the  even-granular 
texture  may  be  found.  The  porphyritic  texture  is 
extremely  common  in  lavas  and  in  intrusives  of  small 
mass  such  as  dikes,  sheets  and  laccoliths;  it  is  rarer  in 

*  The  porphyritic  texture  is  not  a  contrast  of  colors  of  mineral 
grains  but  of  sizes.  Care  must  be  taken,  therefore,  not  to  confuse, 
for  instance,  a  white  rock  consisting  of  grains  of  light-colored  minerals 
such  as  feldspar,  in  which  are  embedded  a  few  conspicuous  black 
grains  of  a  ferromagnesian  mineral  of  the  same  size,  such  as  horn- 
blende, with  a  porphyry. 


PLATE  7. 


A.    With  Phenocrysts  of  Feldspar. 


B.    With  Phenocrysts  of  Augite. 
PORPHYRY   TEXTURE. 


GENERAL  PETROLOGY  OF  IGNEOUS  ROCKS      157 

the  abyssal  rocks,  but  is  sometimes  seen,  especially  in 
granites. 

Origin  of  Porphyritic  Texture.  In  the  case  of  many 
effusive  rocks  or  lavas  it  is  easy  to  understand  why  they 
have  a  porphyritic  texture.  The  lavas  of  many  volcanoes, 
as  they  issue  to  the  outer  air,  are  full  of  growing  crystals, 
often  of  considerable  size,  suspended  in  the  molten  fluid. 
The  latter,  however,  subjected  to  new  conditions,  is 
forced  to  cool  rapidly  and  assumes  a  fine-grained,  or 
dense  crystalline,  or  even  a  glassy,  solid  condition  with 
these  larger  crystals  embedded  in  it,  and  thus  the  com- 
pleted rock  has  a  porphyritic  fabric.  The  same  process 
may  serve  to  explain  this  texture  in  some  of  the  smaller 
intrusives,  such  as  dikes  and  sheets,  but  it  cannot  serve 
as  a  general  explanation  for  all  cases  because  in  some 
dikes,  laccoliths,  etc.,  there  is  good  evidence  that  the 
phenocrysts  have  not  been  brought  thither  but  have 
formed,  like  the  rest  of  the  rock,  in  the  place  where  we 
now  find  them.  It  also  fails  to  explain  the  porphyritic 
border  of  many  granites  and  the  large  phenocrysts  found 
in  other  granites;  nor  does  it  explain  the  origin  of  the 
phenocrysts  themselves,  why  a  few  large  crystals  have 
formed  while  the  rest  of  the  magma  fails  to  crystallize. 
Evidently  some  more  general  explanation  is  needed. 

It  has  been  previously  shown  that  molten  magmas 
must  be  considered  as  strong  or  saturated  solutions  of 
some  compounds  in  others.  As  the  mass  cools  down  it 
may  become  supersaturated.  Now  it  has  been  shown 
that  some  saturated  solutions  cannot  crystallize  spon- 
taneously but  require  to  be  inoculated  with  a  minute 
fragment  of  the  substance  in  solution;  this  is  called  the 
metastable  state.  Other  saturated  or  supersaturated 
solutions  either  crystallize  spontaneously  or  can  be 
induced  to  do  so  by  shaking  or  stirring  with  a  foreign 
substance. 

Miers  has  shown  that  the  same  solution  may  pass  from 
one  to  the  other  of  these  states  in  accordance  with  changes 


158  ROCKS  AND  ROCK  MINERALS 

of  temperature,  and  suggests  that  a  magma  may  be  in  the 
metastable  condition  in  which  a  relatively  few  crystals 
induced  by  inoculation  from  the  surrounding  rocks  are 
growing  as  phenocrysts  and  by  cooling  pass  into  the  labile 
condition  when  spontaneous  crystallization  of  the  remain- 
ing liquid  will  ensue  and  form  the  groundmass.  Or  it 
may  start  in  the  labile  condition  when  the  formation  of  a 
crop  of  phenocrysts  will  reduce  it  to  the  metastable  state, 
in  which  condition  it  may  be  erupted  as  a  lava,  or  remain- 
ing and  cooling  down  it  may  pass  into  a  new  labile  state, 
thereupon  crystallize  and  form  the  groundmass.  The 
recognition  of  these  states  in  cooling  saturated  solutions 
(and  we  must  regard  the  molten  magmas  as  such)  seems 
quite  sufficient  to  explain  the  different  variations  of  por- 
phyritic  texture  which  occur. 

Some  Structures  of  Igneous  Rocks. 

The  word  texture  is  reserved  for  those  appearances  of 
the  rocks  which  are  occasioned  by  the  size,  shape,  color, 
etc.,  of  the  component  crystal  grains.  Certain  larger 
features  exhibited  by  the  rocks  may  be  classed  under  the 
term  of  structure  and  will  now  be  described.* 

Vesicular  Structures.  When  a  molten  magma  rises  to 
the  surface  and  especially  if  it  issues  in  the  form  of  lava, 
the  pressure  upon  it  is  relieved  and  the  water  and  other 
vapors  it  may  contain  are  given  off.  This  has  a  tendency, 
if  it  is  still  soft  and  stiffening,  to  puff  it  up  into  spongy 
vesicular  forms  as  illustrated  in  Plate  8.  In  the  case 
of  very  siliceous  lavas  it  may  be  entirely  changed  into 
a  light  glass  froth  called  pumice.  Such  forms  are  espe- 
cially produced  in  the  lava  in  the  throat  of  a  volcano, 
where  the  issue  of  gases  is  rapid  or  in  the  top  portion  of  a 
flow.  Except  in  a  rare  and  very  limited  way  on  the  sides 

*  An  example  of  the  difference  between  the  two  usages  would  be 
this.  A  certain  lava  from  flowage  might  appear  in  layers;  the 
layers  are  of  rock  composed  of  exceedingly  fine  particles.  We 
would  say  then  that  the  lava  had  a  banded  structure  and  a  very 
fine  compact  texture. 


PLATE  8. 


A.    VESICULAR    LAVA. 


B.    AMYGDALOIDAL    BASALT. 


GENERAL  PETROLOGY  OF  IGNEOUS  ROCKS     159 

of  dikes  they  never  occur  in  intrusive  rocks,  and  the 
presence  of  well-marked  vesicular  structure  may  be  taken 
as  pretty  sure  evidence  that  the  rock  exhibiting  it  was 
originally  a  surface  lava.  In  the  throat  of  a  volcano  such 
spongy  forms  of  lava  may,  by  explosions  of  steam,  be 
driven  in  fragments  into  the  air  to  fall  as  dust,  ashes, 
lapilli,  etc.,  making  volcanic  tuffs  and  breccias  as  described 
elsewhere. 

Amygdaloidal  Structure.  Amygdaloid.  When  a  lava 
has  been  rendered  spongy  (vesicular,  as  described  above), 
it  may  be  permeated  by  heated  waters  carrying  material 
in  solution  which  may  be  deposited  as  minerals  in  the 
cavities.  This  happens  especially  in  basaltic  lavas,  and 
the  dark  rock  then  appears  filled  with  round  or  ovoid 
whitish  bodies  which  from  a  fancied  resemblance  to  the 
kernel  of  a  nut  are  termed  amygdules,  from  the  Greek  word 
for  the  almond.  The  structure  is  called  the  amygdaloidal 
and  a  rock  exhibiting  it  is  often  termed  an  amygdaloid. 
It  is  shown  in  Plate  8.  While  the  smaller  cavities  are 
usually  filled  solid,  the  larger  ones  are  often  hollow,  the 
minerals  projecting  in  crystals  from  the  walls  as  in  geodes, 
and  from  such  amygdaloidal  cavities  some  of  the  most 
beautiful  crystallizations  are  obtained.  The  minerals 
most  frequently  occurring  are  quartz,  which  is  sometimes 
of  the  amethyst  variety,  calcite  and  particularly  zeolites. 
The  basaltic  lavas  are  an  especial  home  of  these  latter 
minerals,  some  of  the  more  common  kinds  being  analcite, 
stilbite,  natrolite,  heulandite  and  chabazite.  The  basalts 
of  India,  Iceland,  Scotland,  Nova  Scotia  and  other 
localities  have  furnished  specimens  which  are  known  in 
all  mineral  collections. 

This  structure  is  most  commonly  and  typically 
developed  in  surface  lavas,  that  is,  in  effusive  rocks,  but 
it  is  also  seen  at  times  in  intrusive  rocks,  such  as  dikes 
and  sheets,  especially  at  their  margins. 

Miarolitic  Structure  and  Porosity.  The  volume  which 
a  magma  occupies  in  the  molten  condition  is  considerably 


160  ROCKS  AND   ROCK  MINERALS 

greater  than  that  which  it  has  when  changed  to  a  solid 
crystalline  rock.  It  is  probably  greater  in  the  liquid  state 
than  when  cooled  to  a  glass  but  how  much  we  do  not  know. 
This  contraction  in  volume,  in  passing  into  the  crystalline 
state,  is  accompanied  by  a  corresponding  rise  in  specific 
gravity.  Thus  an  obsidian  glass,  consisting  chiefly  of 
high  silica  with  moderate  amounts  of  alkalies  and  alumina, 
has  an  average  specific  gravity  of  about  2.2-2.3,  but  the 
same  material  crystallized  into  a  quartz-feldspar  rock 
(granite)  has  a  specific  gravity  of  2.6-2.7.  There  would 
be  a  corresponding  reduction  in  volume. 

In  general  this  contraction  of  volume,  during  the  process 
of  crystallizing,  produces  minute  interspaces  or  pores 
between  the  mineral  grains,  and  cracking  and  jointing  of 
the  mass,  a  process  described  in  the  following  section. 
This  production  of  pores  accounts  for  the  capacity  of  the 
rocks  to  absorb  moisture.  It  appears  to  be  greatest  in 
the  coarse-textured  rocks,  much  less  in  the  finer-grained 
ones;  greater  in  granites,  less  in  diorites  and  other  ferro- 
magnesian  rocks.  In  the  case  of  porous  vesicular  lavas  the 
amount  of  pore  space  may  be  very  great,  but  in  ordinary 
crystalline  igneous  rocks  it  is  small,  usually  less  than  one 
per  cent  of  the  rock  volume. 

In  some  cases,  however,  there  may  be  distinct  cavities 
produced.  These  are  commonly  very  small,  sometimes 
an  inch  or  so  in  diameter  and  in  rare  instances  as  much 
as  several  feet.  It  often  happens  that  the  crystal  com- 
ponents of  the  rock  on  the  boundary  walls  of  the  cavity 
are  much  larger  in  size  than  the  average  grain,  and  project 
into  it,  bounded  by  distinct  faces  and  of  good  crystal  form. 
One  notices  also,  especially  in  granites,  that  the  quartz 
and  feldspar  crystals  are  often  accompanied  by  those  of 
muscovite,  topaz,  tourmaline  and  others  which  are  foreign 
to  the  general  mass  of  the  rock  but  are  common  in  peg- 
matite veins.  These  are  also  well  crystallized.  The 
presence  of  the  water  vapor,  fluorine,  boron,  etc.,  necessary 
for  their  production,  as  well  as  the  larger  size  and 


PLATE  9. 


A.    MIAROLITIC    CAVITY. 


B.    MIAROLITIC   CAVITIES   PASSING    INTO    PEGMATITE. 


GENERAL  PETROLOGY  OF  IGNEOUS  ROCKS     161 

distinct  form  of  the  crystals,  shows  that  such  mineralizers, 
excluded  elsewhere  from  the  magma  during  the  process 
of  crystallization,  collected  in  these  cavities,  possibly 
helped  to  enlarge  them,  and  promoted  the  formation  of 
the  unusual  minerals  and  the  good  crystal  forms  which 
they  and  the  ordinary  rock  minerals  exhibit.  Such 
hollow  spaces  are  called  miarolitic  cavities  and  a  rock 
which  contains  them  is  said  to  have  miarolitic  structure, 
from  a  local  Italian  name  (miarolo)  for  the  Baveno 
granite  which  shows  it.  Such  drusy  cavities  are  distin- 
guished from  geodes  and  others,  in  which  the  minerals 
have  been  deposited  from  solutions,  by  the  fact  that  they 
have  no  distinct  wall  separating  the  minerals  from  the 
containing  rock.  They  often  furnish  fine  mineral  speci- 
mens. An  example  of  one  is  seen  in  Plate  9. 

Jointing  of  Igneous  Bocks.  The  most  important  way 
in  which  the  contraction  of  a  body  of  magma,  after 
cooling  and  crystallizing  into  rock,  manifests  itself  is  in 
the  production  of  joints.  These  are  the  cracks  or  fissures 
which,  running  in  various  directions,  divide  the  mass  into 
blocks,  fitted  together  like  masonry  and  usually  according 
to  more  or  less  definite  systems.  Sometimes  this  shows 
itself  in  the  formation  of  rudely  cubic  or  rhomboidal 
blocks,  as  shown  in  granites  and  other  abyssal  rocks, 
sometimes  in  a  platy  parting  which  may  be  quite  thin  and 
cause  the  rock  mass  at  first  glance  to  resemble  sedimentary 
beds,  and  sometimes  in  concentric  or  spheroidal  forms 
which  develop  rounded  or  ovoid  bodies  like  melons  as 
the  weathering  and  rock  decay  progresses.  Platy  and 
spheroidal  partings,  and  jointing  on  a  small  scale  by  which 
the  rock  body  is  divided  into  little  blocks,  are  most  com- 
mon in  small  intrusions  —  in  dikes,  sheets,  etc.  —  and  in 
surface  lavas.  Such  jointing  is  a  matter  of  great  geologic 
importance  in  permitting  the  entrance  of  air  and  water 
to  act  in  the  weathering  and  decay  of  rocks  and  in  the 
processes  of  erosion,  especially  the  splitting  and  breaking 
of  them  by  the  action  of  frost.  As  can  be  readily 


162        ROCKS  AND  ROCK  MINERALS 

inferred  it  is  also  of  great  practical  importance  in  the 
work  of  rock  excavation,  in  mining  operations  and  in 
quarrying.  (See  Plate  10.)  Were  it  not  for  such  joints 
almost  every  igneous  rock  mass  would  furnish  suitable 
material  for  quarrying,  whereas  on  the  contrary  it  is  diffi- 
cult to  find  a  granite  jointed  on  so  large  a  scale  that  it 
will  furnish  solid  blocks,  for  example,  like  those  from 
which  the  celebrated  Egyptian  obelisks  were  made. 

Columnar  Structure.  The  most  remarkable  way  in 
which  the  jointing  of  a  cooling  mass  of  igneous  rock, 
explained  above,  manifests  itself  is  in  the  production  of 
columnar  structure.  This  is  found  both  in  intrusive  and 
extrusive  occurrences  and  in  all  kinds  of  igneous  rocks,  but 
is  usually  best  displayed  in  basalts.  The  whole  mass  is 
made  up  of  columns,  regularly  fitted  together,  from  a  few 
inches  to  several  feet  in  diameter  and  from  one  foot  to  two 
hundred  feet  or  even  more  in  length.  An  example  is 
shown  in  Plate  11.  The  celebrated  Giant's  Causeway  on 
the  north  coast  of  Ireland  is  one  of  the  best  known 
examples  of  this.  In  the  most  perfect  cases,  as  in  the  one 
just  mentioned,  the  cross  sections  of  the  columns  are 
regular  hexagons  and  the  columns  are  divided  lengthwise 
at  regular  intervals  by  cross  joints  whose  upper  surfaces 
are  shallow  cup-shaped.  The  columns  are  always  per- 
pendicular to  the  greatest  extension  or  main  cooling 
surface  of  the  igneous  mass,  hence  in  a  lava  flow  or  intru- 
sive sheet  they  are  vertical  —  assuming  the  flow  or  sheet 
to  be  horizontal  —  while  in  a  dike  they  tend  to  be  hori- 
zontal. Such  a  dike  when  exposed  by  erosion  tends  to 
resemble  a  stretch  of  cord-wood  regularly  piled. 

The  cause  of  this  structure  seems  to  be  as  follows. 
When  a  homogeneous  mass  is  cooling  slowly  and  regu- 
larly, centers  of  cracking  tend  to  occur  on  the  cooling  sur- 
faces at  equally  spaced  intervals.  From  each  central 
interspace  three  cracks  radiate  outward  at  angles  of  120 
degrees  from  each  other.  These  intersecting  produce 
regular  hexagons  and  the  cracks  penetrating  downward 


PLATE  10. 


A.    High  Isle  Quarry,  Maine. 


B.    Allen  Quarry,  Mount  Desert,  Maine. 

JOINTING    IN   GRANITE   AND    ITS    USE    IN   QUARRYING. 
(U.  S.  Geological  Survey.) 


GENERAL  PETROLOGY  OF  IGNEOUS  ROCKS     163 

make  columns.  This  regular  arrangement  produces  the 
greatest  amount  of  contraction  with  the  least  amount 
of  cracking,  provided  the  centers  are  equally  spaced. 
But  as  the  contractional  centers  are  not  always  equally 
spaced,  three,  four,  five  and  even  seven-sided  columns 
occur.  The  columns  again,  contracting  lengthwise,  break 
into  sections  as  they  form.  The  same  principle  is  also 
seen  in  drying  mud-flats  which  crack  into  polygonal 
shapes  and  in  the  prisms  of  drying  and  contracting  starch. 
Such  columns,  however  regular  their  appearance,  are 
not  crystals  but  pieces  of  rock  and  should  not  be  confused 
with  the  hexagonal  prisms  produced  by  the  crystallization 
of  certain  minerals,  such  as  quartz,  beryl,  etc.,  which  are 
due  to  an  entirely  different  process. 

Inclusions  in  Igneous  Rocks. 

Not  infrequently  there  may  be  noticed  in  igneous 
rocks  masses  which  differ  in  mineral  composition,  color 
and  texture  from  the  rock  which  includes  them.  They 
may  vary  in  size  from  a  fraction  of  an  inch  to  several  yards 
across.  Study  of  them  shows  that  sometimes  they  present 
all  the  characters  of  distinct  kinds  of  rock  and  by  these, 
and  by  their  angular  shapes,  they  show  clearly  that  they 
are  fragments  of  pre-existent  rocks  which  the  magma  on 
its  way  upward  has  torn  loose  from  the  walls  of  its  con- 
duit and  brought  along,  or  blocks  from  the  roof  or  sides  of 
the  chamber,  in  which  the  magma  came  to  rest,  which  were 
loosened  and  sank  into  it.  They  may  be  composed  of 
other  kinds  of  igneous  rocks  or  of  sedimentary  ones,  such 
as  shales,  limestones,  etc.  In  the  former  case  they  are 
not  usually  much  changed,  but  the  fragments  of  stratified 
rocks  generally  exhibit  the  results  of  intense  metamorphic 
action,  such  as  described  elsewhere,  and  are  much  altered. 
In  large  intrusive  masses  inclusions  of  this  character  are 
most  apt  to  occur  near  the  border.  An  inclusion  in 
granite  is  shown  in  Fig.  1,  Plate  12. 

In  other  cases  the  inclusions  are  composed  of  certain 


164  ROCKS  AND   ROCK  MINERALS 

minerals,  especially  the  ferromagnesian  ones,  which  occur 
in  the  rocks  and  which  by  some  process  have  been  aggre- 
gated into  lumps,  such  as  the  masses  of  olivine  crystals 
often  found  in  basalts.  It  is  clear  that  such  aggrega- 
tion or  growth  of  these  minerals  must  have  taken  place 
while  the  remainder  of  the  rock  was  still  in  a  liquid 
condition.  They  have  been  termed  segregations. 

In  still  another  kind  the  inclusions  are  indefinite  in 
form  and  often  of  boundary;  they  are  apt  to  be  drawn  out, 
lenticular,  streaky  in  character  and  they  may  consist  of 
the  same  minerals  as  the  main  mass  of  the  rock  but  in 
quite  different  proportions,  or  they  may  contain  dif- 
ferent minerals.  Thus  one  sees  streaks  in  granite  which 
may  be  much  richer  in  hornblende  or  biotite  than  the 
enclosing  rock.  Some  have  held  that  these  are  due  to 
inclusions  of  other  rocks  which  have  been  melted  up  and 
then  recrystallized  and  in  some  cases  they  may  have  had 
this  origin,  but  for  the  most  part  they  are  regarded  by 
the  majority  of  petrographers  as  caused  by  streaks  and 
spots  in  the  original  magma  of  a  different  chemical  com- 
position from  the  main  portion.  The  cause  of  such  non- 
homogeneousness  in  the  magma  is  ascribed  to  differentia- 
tion, as  discussed  elsewhere  in  this  volume.  Such  streaky 
portions  are  called  by  the  Germans  schlieren  and  in  default 
of  anything  better  this  word  is  often  used  for  them  in 
English.- 

Sometimes  lavas  show  a  streaky  or  even  well  banded 
structure,  portions  differing  from  one  another  in  com- 
position or  in  texture  having  been  drawn  out  in  the 
flowage.  This  is  known  as  the  eutaxitic  structure. 

Origin  of  Igneous  Rocks  —  Differentiation. 

The  fact  that  lavas  differing  decidedly  from  each  other 
in  mineral  and  consequently  in  chemical  composition 
have  been  erupted  by  the  same  volcano  at  different 
periods,  early  attracted  the  attention  of  geologists  and  led 
to  much  speculation  as  to  its  cause.  Thus  felsites  and 


PLATE  11. 


2    D 


GENERAL  PETROLOGY  OF  IGNEOUS  ROCKS      165 

basalts  have  both  been  frequently  noticed  as  the  products 
of  eruption  from  a  single  vent.  One  explanation,  which 
used  to  be  advanced,  was  that  within  the  earth  there  were 
two  layers  of  magma,  an  upper  one  rich  in  silica,  alumina 
and  alkalies,  the  other  and  lower,  poor  in  silica  but  rich 
in  iron  and  magnesia;  accordingly  as  the  eruption  came 
from  one  or  the  other  of  these,  felsites  or  basalts  were 
produced,  while  their  mixtures  gave  rise  to  intermediate 
products.  It  was  soon  seen,  however,  both  on  chemical 
and  geological  grounds,  that  this  view  was  insufficient  to 
explain  the  origin  of  all  igneous  rocks. 

As  the  study  of  rocks  progressed,  other  facts  of  a  similar 
nature  came  to  light.  Thus  in  the  single  rock  mass  com- 
posing the  core  or  neck  of  an  old  volcano,*  where  the 
magma  cooled  under  conditions  favorable  for  the  pro- 
duction of  the  even-granular  or  granitic  texture,  it  is  not 
infrequent  to  find  that  it  is  composed  of  two  or  more 
distinct  kinds  of  rock.  The  boundary  between  these 
will  sometimes  show  that  one  was  erupted  after  the  other 
had  solidified  in  its  place,  since  fragments  of  the  latter  are 
enclosed  in  the  former.  This  is  of  course  merely  carrying 
deeper  down  into  the  volcanic  conduit  the  same  facts 
shown  by  the  surface  lavas  previously  mentioned.  Other 
cases,  however,  are  of  a  different  nature  and  of  such 
geological  importance  that  they  demand  separate  con- 
sideration. 

Border  Zones.  In  recent  years  the  study  of  deep-seated 
intrusive  masses,  such  as  stocks  of  granite,  syenite,  etc., 
which  have  become  exposed  by  long  continued  erosion, 
has  shown  that  not  uncommonly  such  masses  have  an 
outer  border  or  mantle  of  rock  which  differs  in  mineral 
composition  from  the  mass  which  it  enfolds.  The  thick- 
ness of  such  a  border  zone  is  very  variable,  even  in  the 
same  mass,  and  in  places  it  may  be  lacking;  it  may  be 
several  thousand  feet  thick  or  only  a  few  hundred  or  even 
less.  While  in  general  it  bears  some  proportion  to  the 

*  See  volcanic  necks,  page  138. 


166        ROCKS  AND  ROCK  MINERALS 

general  size  of  the  whole  mass  there  is  no  rule  about  this 
which  can  be  stated. 

In  most  cases  this  zone  or  border  jades,  as  it  is  some- 
times called,  is  produced  by  an  enrichment  of  the  rock 
in  the  ferromagnesian  minerals,  such  as  pyroxene,  horn- 
blende, biotite  and  iron  ore.  Generally  the  enriching 
minerals  are  the  same  as  those  more  sparsely  distributed 
in  the  main  rock  body  but  very  often  different  ones  are 
observed  among  them.  From  this  it  is  clear  that  chemi- 
cally the  border  zone  is  richer  in  iron  and  magnesia,  and 
to  some  extent  in  lime,  than  the  main  mass,  with  a  corre- 
sponding diminishing  of  silica,  alumina  and  alkalies. 
Since  they  contain  less  of  silica,  the  acid  oxide,  they  are 
commonly  called  basic  zones.  Not  all  border  zones, 
however,  are  basic  ones;  a  number  of  instances  are  known 
where  the  margin  of  the  intrusion  is  poorer  in  lime, 
iron  and  magnesia  and  consequently  in  ferromagnesian 
minerals  than  the  interior  rock  body  and  therefore  con- 
tains more  silica,  alumina  and  alkalies,  which  expresses 
itself  mineralogically  in  greater  abundance  of  feldspar 
and  sometimes  of  this  and  quartz.  In  this  case  they  are 
called  acid  border  zones.  Thus  on  the  one  hand  intrusions 
of  syenite  have  been  found  which  pass  into  pyroxenite  at 
the  border  while  on  the  other  hand  syenite  intrusions  are 
known  which  become  granite  towards  the  margin.  It 
must  not  be  imagined  that  there  is  anything  approaching 
a  contact  between  the  two  kinds  of  rock.  The  one  kind 
passes  gradually  into  the  other  without  change  in  texture 
and  all  the  facts  indicate  that  this  arrangement  was  not 
produced  by  successive  intrusions  of  different  magmas 
but  by  some  process  in  a  single  body  of  magma  after  it 
had  entered  into  its  chamber. 

Zoned  Laccoliths.  The  zonal  arrangement  just  men- 
tioned is  still  more  strikingly  shown  in  the  case  of  certain 
laccoliths  which  have  been  found  in  Montana  and  else- 
where. Where  these  have  been  laid  bare  and  dissected 
by  erosion  the  study  of  them  shows  that  they  consist  of  a 


GENERAL  PETROLOGY  OF  IGNEOUS  ROCKS       167 

body  of  rock  of  one  kind,  generally  one  consisting  mostly 
of  pyroxene,  enclosing  within  a  core  of  rock  of  a  totally 
different  kind,  usually  a  syenite,  which  is  of  course  chiefly 
feldspar.  A  cross  section  through  such  a  laccolith  is 
shown  in  the  accompanying  diagram,  Fig.  70. 


6   . 


Fig.  70.  Diagram  of  a  Zoned  Laccolith:  a,  feldspar  rock;  6,  pyroxene  rock; 
c,  shales  and  sandstone ;  d,  underlying  sheet  of  intrusive  basalt.  Figures 
in  feet  are  heights  above  sea-level. 

That  the  pyroxenic  rock  once  had  the  extension  shown 
by  the  restoration  in  the  figure  is  known  from  other 
examples  in  the  neighborhood  where  the  erosion  has  not 
been  so  great,  and  it  is  still  found  above,  enwrapping  the 
interior  syenite. 

Associated  Complementary  Dikes.  Another  phenom- 
enon, of  the  same  category  as  those  just  described,  is  seen 
in  the  dikes  so  commonly  found  associated  with  larger 
intrusive  bodies,  such  as  stocks  of  granite,  syenite,  diorite, 
etc.,  where  these  have  become  exposed  by  dissective 
erosion.  They  are  in  origin  subsequent  to  the  main  mass 
which  they  accompany  and  are  found  cutting  it  and  also 
the  surrounding  rocks.  In  the  latter,  these  minor  intru- 
sions may  appear,  not  only  in  the  form  of  dikes,  but  also 
in  intrusive  sheets,  laccoliths,  etc.  These  rocks  are 
divisible  into  two  classes;  in  the  first  they  are  very  poor 
or  entirely  wanting  in  ferromagnesian  minerals  (salic 
rocks)  and  have  been  called  aplitic  dikes,  since  the  dikes 
of  aplite  usually  found  associated  with  granites  are  the 
most  common  and  best  known  representatives  of  this 
class.  They  have  also  been  called  leucocratic  dikes  (from 
the  Greek,  prevailing  white)  in  allusion,  to  their  general 
light  color,  due  to  the  fact  that  they  are  mostly  composed 
of  feldspars  or  of  these  with  quartz.  They  are  generally 


168  ROCKS  AND  ROCK   MINERALS 

fine-grained  rocks,  sometimes  of  a  sugar  granular  texture, 
sometimes  dense  and  to  be  classed  as  felsites.  In  some 
cases  they  are  porphyritic.  They  usually  occur  in  narrow 
dikes,  a  few  feet  wide  and  sometimes  only  an  inch  or  even 
less  in  breadth. 

In  the  second  class  the  rocks  are  heavy,  dark  or  even 
black,  of  basaltic  aspect  and  composed  chiefly  of  ferro- 
magnesian  minerals,  iron  ore,  pyroxene,  hornblende, 
biotite  and  olivine,  in  variable  amounts  and  with  very 
subordinate  feldspar.  They  are  very  commonly  por- 
phyritic with  good-sized  phenocrysts  of  the  minerals 
mentioned  above  in  a  dense  dark  groundmass,  though 
these  are  often  wanting.  Such  rocks  have  been  called 
lamprophyres  (from  the  Greek,  meaning  glistening  por- 
phyry in  allusion  to  the  biotite),  and  are  termed  melano- 
cratic  rocks  (/teAavos,  black) .  In  our  field  classification 
they  would  be  named  biotite  melaphyre  (or  mica  trap), 
hornblende  melaphyre,  etc.,  according  to  the  prevailing 
phenocrysts.  They  also  usually  occur  in  narrow  dikes 
and  are  more  apt  to  cut  the  surrounding  rocks  than  the 
main  intrusive  body  they  accompany,  thus  reversing  the 
custom  of  the  aplites. 

These  two  kinds  of  rocks,  the  aplitic,  light-colored 
feldspathic,  and  the  lamprophyric,  dark-colored,  with 
ferromagnesian  minerals,  are  termed  complementary 
because  taken  together  they  represent  the  composition  of 
the  main  masses  they  accompany.  If  we  could  mix  them 
in  amounts  proportional  to  the  bulk  of  their  occurrence 
we  should  obtain  a  rock  whose  chemical  (and  largely 
mineral)  composition  would  be  that  of  these  larger  masses 
upon  which  they  appear  to  depend  as  satellite  bodies. 
In  some  cases  this  has  been  actually  tested  and  proved. 
When  all  the  facts  concerning  their  mode  of  occurrence  are 
taken  into  account  they  appear  to  have  been  formed  by 
secondary,  later  intrusions  of  the  same  magma  producing 
the  larger  stocks,  which  in  some  way  has  divided  into 
two  unlike  sub-magmas.  If  they  should  break  through 


GENERAL  PETROLOGY  OF  IGNEOUS  ROCKS     169 

to  the  surface  they  would  give  rise  to  lava  flows  also 
unlike,  to  felsites  and  basalts,  and  thus  explain  in  part  the 
phenomena  noticed  in  many  volcanoes. 

It  is  to  be  understood  of  course  that  not  all  dikes, 
sheets  and  laccoliths  belong  in  this  category  of  com- 
plementary rocks.  On  the  contrary  we  very  often  find 
that  the  same  magma  which  produces  stocks,  necks, 
etc.,  occurs  in  intrusions  of  this  character.  They  then 
have  the  same  minerals  and  composition  as  the  larger 
masses,  or  if  independent  bodies  they  usually  contain  both 
ferromagnesian  and  feldspathic  minerals  in  due  amounts. 
Only,  as  explained  on  page  153,  they  are  liable  to  differ 
in  texture  from  the  stocks  and  are  very  apt  to  be  por- 
phyries. Dikes,  etc.,  of  this  kind  have  been  called 
aschistic,  which  means  undivided,  while  the  complementary 
aplites  and  lamprophyres  have  been  termed  diaschistic, 
which  means  divided,  in  allusion  to  their  dual  nature. 

Differentiation.  The  varied  lavas  of  volcanoes,  the 
marginal  zones  of  stocks  and  necks,  the  zoned  laccoliths 
and  the  associated  complementary  rocks,  which  have  been 
described  in  foregoing  sections,  as  well  as  other  similar 
features,  present  to  us  a  body  of  geological  facts  that  can 
only  be  satisfactorily  explained  by  the  assumption  that 
in  some  way  magmas,  which  form  igneous  rocks,  have 
the  capacity  of  separating  into  sub-magmas,  unlike  the 
original,  but  which,  if  mixed  in  proper  proportions  to  a 
homogeneous  whole,  would  again  reproduce  it.  Regard- 
ing the  division  there  seems  to  be  in  general  two  opposite 
poles  toward  which  the  sub-magmas  tend;  to  one  con- 
centrate the  iron,  magnesia  and  to  a  large  extent  the  lime, 
to  the  other  the  alkalies,  alumina  and  to  a  great  extent  the 
silica.  The  one  gives  us  ferromagnesian  rocks  such  as  gab- 
bro,  the  other  feldspathic  rocks  such  as  granite.  While 
this  is  so  in  general,  we  find  in  detail  the  process  infinitely 
varied  in  nature;  thus  in  some  places  one  may  observe  a 
division  among  the  alkalies,  an  enrichment  of  potash 
towards  one  pole  as  compared  with  soda  or  vice  versa. 


170  ROCKS  AND  ROCK  MINERALS 

If  the  body  of  magma  has  come  to  rest  in  its  chamber  and 
this  process  of  differentiation  takes  place  and  it  then, 
crystallizing,  solidifies  and  forms  rock,  it  is  evident  that 
such  a  rock  body  will  be  unlike  in  its  different  parts,  and 
marginal  zones,  zoned  laccoliths,  etc.,  will  be  produced;  or 
if  further  movements  occur,  producing  new  intrusions  or 
these  with  extrusions,  then  associated  complementary 
dikes,  sheets  and  lava  flows  may  occur. 

This  division  into  sub-magmas  is  termed  the  differen- 
tiation of  igneous  magmas  and  the  reality  of  it  as  a  process 
seems  well  established  on  geological  grounds  by  a  large 
body  of  facts.  That  in  some  manner  such  a  process 
takes  place  and  on  the  other  hand  the  understanding  of 
how  and  why  it  does  take  place,  are  two  entirely  different 
affairs,  and  while  every  one  who  is  thoroughly  conversant 
with  the  facts  is  obliged  to  admit  the  former,  a  wide 
diversity  of  views,  owing  to  insufficient  knowledge,  pre- 
vails in  regard  to  the  latter.  Some  phases  of  this  subject 
are  discussed  in  the  following  paragraphs. 

Formation  of  Zones  and  Ore  Bodies.  One  partial 
explanation  that  has  been  offered  for  the  zoned  structures 
previously  mentioned  is  of  importance  because  it  affords 
at  the  same  time  an  understanding  of  the  origin  of  a 
certain  class  of  ore  bodies  which  in  some  places  are  of 
considerable  extent  and  value.  On  page  148  it  was  shown 
that  there  was  a  general  order  of  crystallization  of  rock 
minerals  beginning  with  the  iron  ores,  then  passing  into 
the  ferromagnesian  silicates  and  finishing  with  the  feld- 
spars and  quartz.  In  an  enclosed  body  of  magma, 
crystallization  would  generally  begin  when  the  tempera- 
ture had  fallen  to  the  proper  degree.  This  would  natu- 
rally first  occur  at  the  outer  walls  where  the  effect  of 
cooling  is  felt.  Against  these  the  iron  ores  and  ferro- 
magnesian minerals,  the  earliest  to  crystallize,  would 
form  and,  if  the  process  were  extremely  gradual,  slow 
convection  currents  in  the  magma  would  bring  fresh 
supplies  of  material  to  crystallize  there  until  large 


GENERAL  PETROLOGY   OF  IGNEOUS  ROCKS      171 

amounts  of  these  minerals  had  formed.  This  might  go 
on  until  the  temperature  had  fallen  to  a  point  where  the 
main  body  of  magma  was  compelled  to  solidify  and  the 
rock  mass  as  a  whole  produced.  The  outer  margin  would 
be  much  enriched  in  the  earlier  formed  minerals,  giving 
a  zoned  arrangement  to  the  whole  mass.  In  such  places 
at  the  margin  the  iron  ores  are  sometimes  so  locally  con- 
centrated as  to  yield  workable  deposits  of  value,  though 
very  commonly  the  ore  is  titaniferous  and  therefore  can- 
.  not  be  used  commercially.  The  same  explanation  has 
been  offered  for  the  occurrence  of  sulphide  ores  of  iron 
containing  copper  and  nickel,  of  corundum  and  of  other 
useful  minerals  found  in  similar  situations. 

Origin  of  Salic  Border  Zones.  The  explanation  given 
above  would  show  how  marginal  zones  richer  in  ferro- 
magnesian  minerals  might  arise  but  it  has  been  observed 
that  masses  of  granitic  and  syenitic  rock  are  sometimes 
poorer  or  deprived  of  these  minerals  at  the  margin  of  the 
mass  while  the  main  part  contains  them  in  considerable 
amounts,  thus  making  salic  zones.  An  explanation  which 
has  been  offered  for  this  is  as  follows:  If  a  solution  of 
a  salt  in  a  liquid  (such  as  sea-water)  be  cooled  down  until 
it  is  forced  to  crystallize  (freeze)  it  is  found  that  the  sub- 
stance in  greatest  excess,  salt  or  liquid,  will  solidify  first 
until  a  certain  definite  proportion  of  dissolved  salt  and 
liquid  are  obtained,  called  the  eutectic  mixture,  when  both 
remaining  salt  and  liquid  will  crystallize  simultaneously 
and  the  whole  mass  become  solid.  The  proportion  of  salt 
to  fluid,  forming  the  eutectic,  varies  with  the  kind  of  salt 
and  of  solvent.  Thus  when  sea  water  freezes  the  ice  first 
formed  contains  no  salt,  the  latter  forming  in  the  remain- 
ing water  a  brine  of  increasing  strength  until  the  eutectic 
point  is  reached,  when  both  solidify  together.  In  the 
case  of  granite  and  syenite  the  oxides  composing  the 
quartz  and  feldspars  are  present  in  great  excess  and  may 
be  considered  the  solvent  for  the  others.  It  is  possible 
that  under  proper  conditions  these  might  solidify  at  the 


172        ROCKS  AND  ROCK  MINERALS 

outer  margin,  the  other  oxides,  those  of  iron,  magnesia, 
etc.,  concentrating  in  the  remaining  portion  and  tending  to 
make  an  eutectic  mixture.  Thus  when  the  whole  solidifies 
the  inner  part  will  contain  ferromagnesian  minerals,  and 
the  outer  part  will  be  poor  or  wanting  in  them.  In  the 
case  of  many  diorites  and  gabbros,  where  the  oxides  of  iron 
and  magnesia  are  in  great  excess,  they  would  be  the  sol- 
vent, and  we  should  expect  border  zones  of  ferromagnesian 
minerals.  It  is  evident  this  explanation,  and  the  one 
previously  given,  which  depends  on  the  order  of  crystal- 
lization, in  the  case  of  highly  feldspathic  rocks,  are 
opposed  to  each  other;  the  first  tends  to  make  ferromag- 
nesian zones  around  granite  and  syenite,  the  latter  salic 
ones.  In  the  diorites  and  gabbros  both  tend  to  produce 
margins  richer  in  ferromagnesian  minerals. 

Zones  by  Absorption.  It  has  also  been  suggested  that 
such  zones  are  produced  by  the  magma  melting  its  con- 
taining walls  and  thus,  by  absorbing  foreign  material, 
becoming  in  composition,  at  its  border,  unlike  the  main 
mass.  Being  thus  unlike  it  would  naturally  have  a 
different  mineral  composition  on  solidification.  It  is 
possible  that  this  may  have  happened  in  some  cases  but 
it  cannot  serve  as  a  general  explanation  because  in  many 
cases  we  find  the  border  of  an  entirely  different  mineral 
(and  chemical)  composition  from  that  which  it  ought  to 
have  if  the  rocks  with  which  it  came  in  contact  had  been 
melted  and  absorbed. 

General  Explanation.  It  is  obvious  that  the  hypotheses 
discussed  above,  while  they  may  serve  to  explain  border 
zones  and  marginal  ore  deposits,  do  not  give  a  general 
explanation  for  the  differentiation  of  igneous  rocks.  For 
the  occurrence  of  complementary  dikes,  of  different  lavas 
from  the  same  volcano,  and  the  mixtures  of  different 
types,  which  are  not  marginal,  in  the  same  stock,  as  well 
as  other  facts,  show  clearly,  that  in  general,  differentiation 
is  not  a  division  by  a  process  of  solidification,  but  one 
which  occurs  in  a  magma  in  such  a  manner  as  to  produce 


GENERAL  PETROLOGY  OF  IGNEOUS  ROCKS      173 

separate  bodies  of  differing  liquids  which  may  be  inde- 
pendently ejected  or  intruded.  It  must  occur  before 
there  is  any  solidification.  While  we  see  that  this  is  so, 
both  from  geological  and  chemical  facts,  no  general 
explanation,  which  is  in  all  respects  satisfactory,  has  been 
offered  for  this  process.  Different  hypotheses,  which  it 
would  be  beyond  the  limits  of  this  work  to  state  and 
discuss,  have  been  suggested  by  various  authorities,  but 
our  knowledge  of  the  physical  chemistry  of  molten  magmas 
is  yet  too  limited  to  know  their  proper  value  and  appli- 
cability. It  is  probable  the  processes  of  differentiation 
are  quite  complex  and  that  they  are  produced  by  a 
variety  of  factors,  the  laws  governing  which  must  all  be 
taken  into  account  in  any  general  explanation.  It  is 
known  that  molten  artificial  glasses  and  molten  alloys 
of  metals,  under  conditions  not  yet  well  known,  do  not 
remain  homogeneous  but  undergo  a  kind  of  differentiation, 
and  it  is  along  this  line  of  experimental  research  that 
light  must  be  sought  to  explain  the  facts  as  we  find  them 
in  Nature. 

Petrographical  Provinces.  Consanguinity.  It  has  been 
noticed  in  the  study  of  rocks,  that  those  belonging  to 
certain  regions  have  particular  features  which  to  a  greater 
or  less  degree  are  found  to  be  distinctive  of  all  the  members 
of  the  group  which  occur  there.  This  is  shown,  sometimes 
in  the  presence  of  particular  varieties  of  minerals,  some- 
times in  peculiar  textures,  sometimes  in  peculiarities  of 
chemical  composition  and  usually  in  a  combination  of 
these  things.  They  may  be  shown  in  varying  degrees  by 
all  the  different  rocks  of  the  region:  thus,  for  example,  by 
syenites  which  are  chiefly  composed  of  feldspar  and  by 
dolerites  in  which  ferromagnesian  minerals  prevail;  in  in- 
trusive stocks  of  granular  rocks  with  their  associated  com- 
plementary dikes  and  sheets  and  in  lava  flows  of  felsites 
and  basalts.  These  common  characters  are  sometimes 
strongly  marked  and  at  other  times  only  to  be  seen  by 
the  experienced  observer.  The  fact  that  such  distin- 


174  ROCKS  AND  ROCK  MINERALS 

guishing  features  occur  in  the  different  types  of  a  certain 
region  and  serve  to  indicate  their  relationship  to  one 
another  and  to  show  a  common  origin  by  differentiation  is 
termed  the  consanguinity  of  igneous  rocks,  and  that  region 
over  which  the  rocks  thus  show  genetic  relations  is  called 
a  petrographic  province,  or  comagmatic  region.  Thus  the 
comagmatic  region  of  South  Norway  is  characterized  by 
the  extremely  high  percentage  of  soda  in  the  magmas, 
which  gives  rise  to  certain  minerals  and  peculiar  rock 
textures ;  those  of  Italy  and  central  Montana  by  very  high 
potash  which  shows  itself  in  the  formation  of  the  mineral 
leucite,  common  in  such  regions  but  rare  or  unknown  else- 
where; that  of  the  western  Mediterranean  islands  and 
eastern  Spain  by  an  abnormally  high  amount  of  titanic 
oxide  in  its  rocks. 

Such  evidences  of  consanguinity  in  rock  groups  and  the 
proofs  which  they  furnish  of  comagmatic  regions  cannot 
usually  be  observed  in  field  work  and  in  the  megascopic 
study  and  determination  of  rocks.  They  generally 
demand  careful  and  complete  investigation  of  thin  sections 
under  the  microscope,  aided  by  chemical  analyses  in  the 
laboratory,  together  with  a  broad  acquaintance  of  the 
literature  of  this  subject,  in  order  to  be  perceived  and 
appreciated.  The  matter,  however,  is  one  of  great  interest 
and  although  one  may  not  be  either  a  chemist  or  petro- 
grapher,  he  may  yet  appreciate  the  significance  of  its 
bearing  on  the  solution  of  problems  of  the  greatest  impor- 
tance in  geology.  It  is  evident  that  before  we  can  safely 
theorize  as  to  the  origin  and  history  of  the  earth  we  must 
first  know  the  nature  of  its  component  parts  and  the  laws 
governing  their  distribution. 

Post-intrusive  Processes, 

When  a  body  of  molten  magma  has  come  to  rest  in  the 
chamber  it  is  destined  to  thenceforth  occupy  as  a  solid- 
ified rock  mass,  cooling  and  eventually  crystallization 
begin.  From  this  point  on,  so  far  as  the  magma  is  con- 


GENERAL  PETROLOGY  OF  IGNEOUS  ROCKS      175 

cerned,  those  factors  are  at  work  which  have  been 
described  elsewhere,  and  which  in  time  will  produce  the 
completed  rock.  During  this  period  of  crystallization 
.the  volatile  substances  dissolved  in  the  magma  and 
previously  contained  under  pressure,  such  as  fluorine, 
boric  acid,  carbon  dioxide  and  especially  and  chiefly 
water,  which  have  been  already  described  as  mineralizers, 
are  gradually  excluded,  except  in  so  far  as  they  may  take 
part  in  the  chemical  composition  of  some  of  the  minerals. 

This  period  in  the  history  of  the  formation  of  a  rock 
body,  when  it  is  solidifying  and  giving  off,  as  it  crystal- 
lizes, heat  and  vapors,  is  called  the  pneumatolytic  (Greek, 
gas,  and  to  loosen),  and  these  agents  generate  important 
results.  At  the  surface  they  give  rise  to  hot  springs, 
fumaroles,  solfataras,  and  similar  secondary  igneous 
phenomena;  in  the  depths  they  produce  in  the  rocks 
surrounding  the  igneous  mass  a  variety  of  features  known 
under  the  term  of  contact  metamorphism,  and  in  the  already 
solidified  parts  of  the  igneous  mass  they  bring  about  the 
formation  of  pegmatite  dikes,  of  greisen  (described  under 
Granite)  and  in  some  cases  of  ore  deposits;  things  which 
are  treated  in  the  following  sections. 

Pegmatite  Dikes  or  Veins.  It  has  been  previously 
stated  that  when  deeply  formed  stocks  or  masses  of  granite, 
syenite,  diorite,  etc.,  have  been  laid  bare  by  erosion  they 
are  very  frequently  found  to  be  cut  by  complementary 
dikes  of  felsitic  and  basaltic  aspect.  In  addition  it  is  also 
frequently  observed  that  they  are  penetrated  by  dikes 
which  display  certain  definite  characters,  the  most  marked 
of  which  is  the  very  large  and  sometimes  enormous  size  of 
the  individual  minerals  composing  them.  Such  dikes 
have  been  termed  pegmatite  dikes,  from  the  name  given 
by  Hauy  to  the  intergrown  masses  of  quartz  and  feldspar 
found  in  them  when  they  occur  in  granites.  Dikes  of 
this  character  not  only  cut  the  stocks  and  batholiths,  to 
whose  intrusion  they  owe  their  origin,  but  are  also  found 
penetrating,  as  offshoots,  the  rock  masses  enveloping 


176  ROCKS  AND  ROCK  MINERALS 

them.  There  are  a  number  of  features  which  particularly 
characterize  them,  as  follows: 

a.  They  consist  in  large  part  of  the  ordinary  minerals 
which  compose  the  rock  to  which  they  belong,  but  these, 
instead  of  having  their  regular  order  of  successive  crys- 
tallization, show  by  their  interpenetration  that  they 
have  crystallized  more  nearly  if  not  entirely,  simul- 
taneously.* The  size  of  the  individual  crystals  is  a 
character  that  has  been  mentioned.  Feldspar  and  quartz 
may  occur  in  crystals  a  foot  or  even  several  feet  long, 
apatite  in  dimensions  like  the  handle  of  a  broom,  mica  in 
crystals  yielding  plates  a  foot  or  more  in  diameter  and 
other  minerals  in  similar  proportions.  It  is  not  to  be 
understood  that  these  sizes  represent  the  average;  they 
are  the  extremes  which  are,  however,  not  infrequently 
attained.  Moreover,  the  essence  of  pegmatite  structure 
does  not  lie  in  mere  size,  for  many  rocks  are  very  coarse- 
grained which  are  not  pegmatites,  but  rather  in  the  other 
qualities  enumerated. 

6.  Another  peculiar  feature  is  that  in  many  pegmatites 
there  is  .an  obvious  tendency  for  the  minerals  to  grow 
outward  from  the  walls  of  the  dike  on  either  side  and 
project  inward  toward  the  center.  This  may  become 
very  marked  and  there  may  even  be  an  empty  space  at  the 
center  into  which  the  minerals  project  showing  crystal 
faces  as  in  miarolitic  cavities  (page  159)  or  in  the  vuggs  of 
mineral  veins.  The  whole  effect  is  to  produce  in  a  rough 
way  a  zoned,  banded  or  ribbon  structure,  which  is  often 
so  perfectly  seen  in  mineral  veins. 

c.  Another  character  is  the  extreme  variability  in  the 
relative  proportions  of  the  component  minerals  from 
place  to  place,  a  variability  not  seen  in  the  main  rock 
mass.  Thus  in  granite  pegmatites  traced  along  the 
outcrop  of  the  dike  great  variations  in  the  relative  amount 
of  quartz  and  feldspar  may  often  be  observed;  in  tracing 

*  See  in  connection  with  this  the  description  of  graphic  granite 
in  the  granite  pegmatite  veins,  p.  212. 


GENERAL  PETROLOGY  OF  IGNEOUS  ROCKS      177 

them  outward  from  the  parent  mass  into  the  enclosing 
rocks  they  may  even  pass  into  pure  quartz  veins.  In 
other  cases  they  may  turn  into  fine-grained  granite 
(aplite)  or  felsite,  and  this  change  in  the  character  of  the 
dike  may  occur  quite  suddenly. 

d.  They  are  very  apt  to  contain  accessory  minerals 
which  are  either  not  found  at  all  in  the  main  rock  mass 
or  which  microscopic  examination  shows  are  sparingly 
distributed  in  very  minute  crystals.  These  minerals  may 
be  divided  into  two  classes.  In  the  one  their  chemical 
composition  shows  that  they  consist  of  the  ordinary 
oxides  which  compose  the  magmas,  alumina,  lime,  iron, 
soda,  etc.,  plus  the  volatile  elements  or  oxides  which  form 
the  mineralizing  vapors.  Common  ones  are  tourmaline, 
which  shows  the  presence  of  boric  acid;  topaz  and  fluorite, 
which  demonstrate  that  fluorine  was  present  and  many, 
of  which  muscovite  mica  is  perhaps  the  most  prominent, 
which  prove  the  important  role  played  by  water  vapor. 
The  other  class  are  characterized  by  their  containing  in 
larger  or  smaller  amounts  the  oxides  of  rare  elements, 
such  as  lithium,  caesium,  beryllium,  molybdenum,  cerium, 
lanthanum,  niobium,  etc.,  elements  which  are  detectible 
with  difficulty  as  minute  traces  or  not  at  all  in  the  main 
rock  mass.  In  combination  with  silica,  titanic  oxide, 
phosphoric  acid,  zirconia,  carbonic  acid,  fluorine,  etc., 
they  give  rise  to  a  whole  host  of  mineral  combinations, 
too  numerous  to  mention,  but  of  which  beryl  and  spodu- 
mene  may  be  cited  as  examples.  No  sharp  distinction 
can  be  drawn  between  these  two  classes;  many  minerals 
might  be  placed  in  either,  but  definite  types  of  both,  like 
those  mentioned,  can  readily  be  found. 

These  accessory  minerals  often  occur  also  in  crystals 
of  great  size  and  sometimes  aggregated  together  in  places 
in  the  dikes  in  very  large  amounts.  It  is  due  to  this  great 
variety  of  minerals  and  the  frequent  size  and  perfection 
of  the  crystals  that  the  pegmatite  dikes  are  mineralogically 
of  great  interest  and  are,  therefore,  a  favorite  hunting 


178  ROCKS  AND  ROCK  MINERALS 

ground  of  mineral  collectors.  It  is  to  be  noted  also  that 
each  variety  of  magma  (or  rock)  is  characterized  by 
special  mineral  combinations  in  its  pegmatite  dikes,  and 
this  applies  not  only  to  the  ordinary  minerals  which  com- 
pose the  rocks  and  distinguish  them  from  one  another 
but  also  to  the  accessory  ones  as  well. 

Thus  the  mineral  groups  found  in  the  pegmatites 
associated  with  ordinary  granites  are  quite  different  from 
those  with  the  nephelite  syenites,  as  these  in  turn  differ 
from  those  of  the  gabbros. 

Origin  of  the  Pegmatites.  The  simultaneous  method  of 
crystallization,  the  arrangements  along  the  walls  of  the 
dike,  the  variability  in  the  proportions  of  the  component 
minerals  and  their  frequent  huge  size  all  show  that  con- 
ditions, different  from  those  which  attended  the  solidifica- 
tion of  the  main  rock  masses,  prevailed  during  the  forma- 
tion of  the  pegmatites.  The  presence  of  hydroxyl,  fluorine, 
boron,  etc.,  also  shows  that  mineralizing  agents  were 
abundantly  present.  Bearing  these  facts,  and  those  of 
the  geologic  mode  of  occurrence,  in  mind,  we  can  present 
to  ourselves  a  view  of  their  origin  which  would  be  some- 
what as  follows: 

When  a  body  of  igneous  magma,  such  as  will  form  a 
stock  or  batholith,  comes  to  rest  in  place  it  will  commence 
to  cool.  This  will  naturally  take  place  first  in  the  upper 
and  outer  portions  and  here  will  begin  the  solidifying  of 
the  mass  by  crystallization.  As  it  becomes  solid  it  breaks 
up  into  jointed  masses  by  contraction.  The  weight  of 
these  masses,  aided  by  the  rock  pressures  from  above  upon 
the  still  liquid  material  below,  tends  to  force  the  latter 
upward  into  the  fissures  in  the  solidified  part  and  into 
those  of  the  surrounding  rocks  and  produce  dikes.  If 
differentiation  is  taking  place  and  there  is  a  concentration 
of  the  iron,  magnesia  and  lime  towards  the  outer  border, 
as  explained  in  previous  sections,  these  dikes  will  be 
complementary  and  we  will  find  aplites  (and  felsites) 
more  commonly  in  the  central  mass,  and  the  corresponding 


GENERAL  PETROLOGY  OF  IGNEOUS  ROCKS      179 

basaltic  lamprophyres  more  commonly  in  the  outer  portion 
and  in  the  surrounding  rocks. 

But  as  the  process  of  crystallization  goes  on,  the  volatile 
substances  in  the  magma,  and  especially  water  in  great 
quantities,  beyond  what  is  retained  by  such  minerals  as 
use  them  in  their  chemical  composition,  will  be  excluded, 
and  more  and  more  as  the  gases  accumulate  they  must 
find  their  way  outward.  Thus  they  will  tend  to  force 
their  way  upward  along  the  fissures  in  the  solidified  parts 
above  and  at  the  sides.  These  cracks  will  therefore  be- 
come channel  ways,  not  only  for  the  still  unconsolidated 
magma  as  mentioned  above  (whether  differentiated  or  not), 
but  also  for  the  vapors  which  will  collect  in  them  and  in 
those  of  the  immediately  surrounding  rock  mantle  under 
pressures,  which  must  often  be  enormous,  until  event- 
ually they  escape.  It  is  evident  from  this  that  the 
ascending  magmas  in  the  fissures  will  at  various  places 
become  supercharged  with  these  vapors  far  beyond  what 
obtains  in  the  normal  rock.  Now,  both  on  experimental 
grounds  and  what  is  observed  in  nature,  it  may  be  re- 
garded as  almost  certain  that  no  sharp  line  can  be  drawn 
between  igneous  fusions  of  silicates  (molten  silicate  mag- 
mas) containing  water  under  pressure  and  hot  water  solu- 
tions. It  appears  that  under  pressure  water  will  mix  in 
all  proportions  with  magma  so  that  at  one  end  are  molten 
fusions,  at  the  other  hot  solutions.* 

At  360  degrees  water  reaches  its  critical  point,  that  is, 
heated  to  this  degree,  or  above,  its  vapor  cannot  be 
turned  back  into  liquid  by  mere  pressure,  however  great 
this  may  be.  At  this  temperature  its  expansive  force  is 
almost  3000  pounds  to  the  square  inch,  which  would 
require  a  vertical  height  of  about  2500  feet  of  granite  rock 
to  contain  it.  Above  this  temperature  sufficient  pressures 
cause  it  to  contract  rapidly  and  it  may  even  occupy  less 

*  A  good  illustration  of  this  is  seen  in  a  solution  of  thallium 
silver  nitrate  which  boils  down,  losing  water,  until  a  clear  molten 
fluid  of  the  double  salt  remains  which  is  anhydrous. 


180  ROCKS  AND  ROCK  MINERALS 

volume  than  it  would  in  the  liquid  state  (see  p.  15). 
The  temperatures  obtaining  in  molten  rock  magmas  are 
far  above  the  critical  temperature  of  water  and  it  must 
therefore  be  in  the  gaseous  condition,  though  under  the 
enormous  pressures  obtaining  under  thousands  of  feet 
or  even  several  miles  vertical  of  overlying  rock,  it  may 
well  be  much  denser  than  water  at  the  surface.  Although 
it  has  not  yet  been  shown,  so  far  as  the  writer  knows, 
that  water  in  this  state  holds  substances  in  solution  just 
as  though  it  were  a  liquid,  we  can  well  imagine  that  at 
high  temperatures  aided  by  the  fluorine  and  other  active 
substances  so  commonly  with  it,  its  solvent  action  must 
be  enormously  increased,  especially  its  ability  to  dissolve 
silica. 

Under  such  conditions  it  is  easy  to  see  that  the  minerals 
would  crystallize  quite  differently  from  those  in  the 
normal  rock;  in  some  places  the  magma  would  be  in 
excess  and  the  results  would  more  nearly  approximate 
those  obtaining  in  the  main  rock;  with  diminished  water 
the  dike  might  pass  into  an  ordinary  aplite  or  felsite 
phase;  with  increased  amounts,  in  another  place,  it 
might  pass  from  the  state  of  a  magma  into  an  aqueous 
solution  and  here  would  be  favorable  conditions  for 
crystallization  on  a  large  scale,  for  growth  outward  from 
the  walls  and  for  the  segregation  of  the  rarer  elements. 
Finally  passing  onward  the  solution  phase  might  become 
more  pronounced,  only  silica  would  be  carried  and  the 
dike  turn  into  a  quartz  vein.  Thus,  as  the  degree  of 
differentiation  of  the  magma  and  the  proportion  of 
magma  to  water  vary,  we  can  see  how  dikes  of  ordinary 
rock,  of  variable  pegmatites  and  quartz  veins  may  be 
formed,  which  show  in  places  very  clearly  their  genetic 
relationships.  Also  the  slow  cooling  that  would  occur  in 
great  masses  of  heated  rock  enclosing  the  fissure  would 
be  favorable  for  the  production  of  large  crystals. 

Contact  Metamorphism.  This  term  is  applied  to  the 
changes  which  are  caused  by  a  body  of  magma  coming  in 


GENERAL  PETROLOGY  OF  IGNEOUS  ROCKS       181 

contact  with  other  rocks  already  formed.  The  word 
metamorphism,  from  the  Greek,  means  a  change  of  form 
or  body  and  is  applied  to  those  results,  induced  by  a 
variety  of  factors,  by  which  rocks  are  recrystallized  with 
the  formation  of  new  minerals  and  textures.  General 
or  regional  metamorphism  by  which  rocks  are  changed 
over  wide  areas  through  various  geological  agencies  is 
considered  in  a  later  chapter;  here  only  the  results  caused 
by  igneous  magmas  are  treated.  In  several  ways  the 
results  of  the  two  are  alike  and  they  often  merge  into  one 
another  but  in  contact  metamorphism  the  extent  of  the 
masses  involved  is,  in  general,  so  much  less  than  in  regional 
metamorphism,  that  from  the  standpoint  of  general  geo- 
logy, it  is  of  much  less  importance.  In  respect  to  petro- 
logy and  to  practical  field  work,  however,  it  is  a  matter 
of  individual  interest  and  great  consequence  and  it  is 
therefore  given  separate  treatment  in  this  place. 

The  effects  of  the  contact  of  a  body  of  magma  with 
other  rocks  is  seen  in  two  ways:  in  one  a  change  from  its 
general  normal  character  is  commonly  observed  in  the 
igneous  rock  itself  and  this  is  termed  the  endomorphic 
effect;  in  the  other,  changes  in  the  rocks  with  which  it  has 
come  in  contact  are  seen  and  this  is  called  the  exomorphic 
effect.  We  will  consider  the  former  one  first. 

Endomorphic  Effects.  It  has  been  stated  in  a  previous 
section  that  a  change  in  the  mineralogical  composition  of 
an  intrusive  rock  body  is  not  infrequently  observed  along 
the  contact,  producing  a  border  zone  or  facies.  This  is 
due  to  a  change  in  the  chemical  composition  of  the  magma, 
caused  by  differentiation,  and  has  been  fully  discussed. 
But  at  times  also,  even  when  this  process  has  not  occurred, 
more  or  less  of  a  change  in  the  minerals  of  the  igneous 
rock  may  be  seen  directly  at  the  contact  or  as  one 
approaches  it,  In  this  case  it  is  due  to  the  presence  of 
mineralizing  vapors  which,  as  previously  described,  tend 
to  be  excluded  as  the  mass  cools  and  crystallizes  and 
to  escape  to  the  margin  and  into  surrounding  rocks. 


182  ROCKS  AND  ROCK  MINERALS 

Through  their  influence  minerals  are  formed  which  do 
not  generally  occur  in  the  main  part  of  the  mass  and  which 
are  those  which  have  been  described  as  characteristic  of 
the  pegmatite  dikes.  In  granites  the  most  characteristic 
is  perhaps  tourmaline,  whose  presence  is  indicative  of 
boron,  hydroxyl  and  fluorine.  It  is  apt  to  take  the  place 
of  the  biotite  in  the  main  rock  and  its  occurrence  as  a 
regular  component  of  the  granite  should  always  lead  to  a 
suspicion  of  approach  to  the  contact,  though  it  is  also 
found  in  the  neighborhood  of  fissures  which  have  served 
as  the  conduit  for  pneumatolytic  exhalations. 

A  variety  of  the  granite  of  the  Black  Hills  from  Harney's 
Peak  illustrates  this  phase;  in  addition  to  the  usual 
quartz  and  feldspar,  the  rock  contains  black  tourmaline, 
abundant  and  well  crystallized  muscovite,  green  beryl 
and  red  garnets:  such  minerals  recall  the  associations 
seen  in  pegmatites. 

It  may  even  happen  that  the  accumulation  of  mineral- 
izing vapors  is  so  great  at  the  outer  margin  before  crystal- 
lization begins  that  the  conditions  are  favorable  there  for 
the  formation  of  a  true  pegmatite  zone.  The  writer  has 
observed  a  granite  stock  in  the  White  Mountains 
enwrapped  by  a  mantle  of  pegmatite;  the  large  plates  of 
muscovite  are  set  perpendicular  to  the  contact  and  the 
mixture  is  much  enriched  in  quartz.  Similar  examples 
are  known  from  other  localities,  and  in  Pelham,  Mass.,  a 
pegmatite  mantle  partly  enfolds  a  mass  of  peridotite  in 
the  gneiss.  Phenomena  of  the  character  described  above 
are  most  noticeable  about  the  larger  intrusions,  such  as 
batholiths,  stocks,  etc.;  in  dikes,  sheets  and  minor 
intrusions  they  are  not  so  conspicuous  or  are  entirely 
wanting. 

A  much  more  common  endomorphic  contact  effect  is  a 
change  in  texture  and  this  is  independent  of  any  change  in 
mineral  composition,  in  fact,  is  largely  observed  where  the 
mineral  composition  remains  constant.  The  most  usual 
feature  of  this  kind  is  a  change  in  the  average  size  of  grain 


GENERAL  PETROLOGY  OF  IGNEOUS  ROCKS      183 

in  the  rock  which  grows  smaller  as  the  contact  is 
approached.  The  rock  indeed  may  become  exceedingly 
dense  at  the  contact  and  thus  for  instance  a  granite  whose 
average  grain  is  of  the  size  of  coarse  shot  may  turn  into 
a  compact  homogeneous  appearing  felsite.  This  is  of 
course  due  to  a  more  rapid  and  general  crystallization 
produced  by  the  chilling  effect  of  the  contact  wall.  In- 
stances are  even  known  where  the  cooling  caused  by  the 
cold  rocks,  with  which  the  magma  came  in  contact,  was 
so  rapid  that  solidification  took  place  at  the  margin  before 
crystallization  could  begin,  with  the  production  of  a  thin 
selvage  sheet  of  glass.  Such  instances  are  most  liable  to 
occur  in  narrow  dikes,  in  which  the  cooling  of  the  con- 
tiguous rocks  is  most  strongly  felt. 

In  other  cases  this  denser  contact  facies  may  contain 
larger  distinct  crystals  or  phenocrysts  and  thus  be  a 
porphyry  while  the  main  mass  is  of  even-granular  texture. 
The  phenocrysts  may  be  anterior  in  origin  to  the  time 
when  the  magma  came  to  rest;  in  the  main  rock  body  they 
may  be  of  the  same  size  as  the  rest  of  the  later  rock  grains 
but  at  the  contact  their  contrast  with  the  later  dense 
material  produces  a  porphyry.  On  the  other  hand,  it 
has  been  observed  that  in  many  intruded  masses  of 
porphyry  occurring  in  dikes  and  sheets  the  phenocrysts 
may  be  entirely  absent  at  the  contact  margin,  and  in  such 
rock  bodies  they  have  been  formed  after  the  period  of 
intrusion,  since  if  they  had  been  brought  up  in  the  ascend- 
ing magma  they  would  be  found  at  the  contact  as  well  as 
in  the  interior  of  the  mass. 

The  cases  treated  above  are  sufficient  to  illustrate  the 
chief  endomorphic  effects  of  contact  metamorphism  in 
igneous  rocks. 

Exomorphic  Effects  of  Contact  Metamorphism  in  General. 
The  effect  of  the  heat  and  vapors  given  off  by  an  intruded 
mass  of  magma  upon  the  surrounding  rocks  with  which 
it  is  in  contact  varies  with  a  number  of  factors.  For  one 
thing  it  naturally  varies  with  the  size  of  the  intruded 


184  ROCKS  AND   ROCK  MINERALS 

mass;  it  also  varies  with  the  nature  of  the  vapors  which 
are  given  off,  as  described  under  pegmatite  formation. 
Another  factor  is  the  nature  of  the  rock  that  is  being 
affected,  some  kinds  being  more  susceptible  than  others, 
and  it  also  depends  on  the  attitude  of  these  rocks,  that 
is,  in  the  case  of  sedimentary  beds,  on  the  position  of  the 
planes  of  stratification  toward  the  igneous  mass.  All  of 
these  are  important  features  and  each  deserves  separate 
treatment  in  order  that  the  subject  may  be  fully  under- 
stood. In  general  it  may  be  said  that  the  most  noticeable 
field  evidence  of  the  exterior  effect  is  a  baking,  hardening 
or  toughening  of  the  surrounding  rocks.  It  not  uncom- 
monly happens  as  a  result  of  this  process  that  they 
resist  erosion  better  than  the  intruded  mass  or  the  un- 
changed enveloping  rocks  and  thus  give  rise  to  distinct 
projecting  topographic  forms.  This  is  admirably  illus- 
trated in  the  Crazy  Mountains  of  Montana,  where  the 
resistant  rocks  of  the  contact  zone  give  rise  to  a  series  of 
high  ridges  and  peaks  which  encircle  a  more  eroded  mass 
of  intruded  igneous  rock  and  rise  sharply  from  a  sloping 
plain  of  soft  unchanged  shales  and  sandstones.  In  the 
case  of  a  dike  it  may  thus  occur  that  the  dike  and  the 
surrounding  beds  are  lowered  more  rapidly  by  erosion, 
while  the  contact  walls  on  either  side  are  left  projecting 
as  two  parallel  ridges. 

The  mineralogical  effect  is  that,  in  general,  where  the 
agencies  have  made  themselves  most  strongly  felt  there 
is  a  recrystallization  of  the  rocks.  This  is  produced  by 
an  interchange  of  the  molecules  within  short  distances 
whereby  former  chemical  combinations  are  broken  up  and 
new  ones  formed.  In  mass,  that  is,  in  sum  total,  the 
chemical  composition  of  the  altered  rock  generally  re- 
mains the  same,  except  that  volatile  compounds,  water, 
carbon  dioxide,  organic  matter,  etc.,  are  driven  out,  and 
in  some  cases,  volatile  components,  fluorine,  boron,  etc., 
may  be  added  by  the  mineralizing  vapors  from  the 
igneous  mass. 


GENERAL  PETROLOGY  OF  IGNEOUS  ROCKS   185 

Modes  of  Occurrence.  The  widest  and  most  pronounced 
contact  zones  as  a  rule  are  noticed  about  the  great  intrusive 
stocks  and  batholiths.  This  is  most  natural,  since  the 
vast  size  of  the  igneous  body  supplies  heat  and  vapors  for 
a  great  length  of  time.  Around  them  contact  zones  a 
mile  and  even  more  in  breadth  have  been  observed  in 
many  places.  Next  to  them  perhaps  the  most  striking 
are  seen  about  old  volcanic  necks.  The  breadth  and 
intensity  here  often  seem  disproportionate  to  the  size  of 
the  igneous  mass  but  this  is  to  be  explained  by  the  fact 
that  the  necks  represent  conduits  through  which  fresh 
supplies  of  highly  heated  matter  have  been  successively 
passing.  This  renewal  of  matter  in  the  conduit  may  thus 
induce  a  superadded  effect.  In  such  cases  there  may  be 
no  endomorphic  effect  of  cooling  on  texture  as  described 
above;  the  conduit  walls  are  so  highly  heated  that  the  tex- 
ture of  the  igneous  rock  remains  the  same  up  to  the  very 
contact  wall. 

In  the  case  of  dikes  considerable  variations  may  be 
seen;  in  small  dikes  the  effect  may  be  noticed  only  a  few 
inches  or  even  less,  while  in  large  ones  it  may  extend 
many  yards  on  either  side.  Again,  some  dikes  have  served 
as  conduits  for  magma  passing  up  through  them  into 
larger  intrusions  above,  feeding  sheets  or  laccoliths  or 
giving  rise  to  extrusive  outflows.  About  them  the  meta- 
morphism  will  naturally  be  greater,  other  things  being 
equal,  than  where  a  fissure  was  filled  by  a  single  charge 
of  magma  which  immediately  came  to  rest.  For  this 
reason  the  metamorphism  induced  by  intrusive  sheets 
and  laccoliths  is  generally  inconsiderable,  since  they  also 
represent  a  single  charge  of  magma  into  the  rocks  about 
them,  which  is  not  renewed.  Immediately  at  the  contact 
and  for  a  few  feet  or  yards  beyond,  the  rocks  may  be 
altered  but  the  effect  soon  dies  out  except  in  the  cases  of 
very  powerful  sheets  and  large  laccoliths.  With  extrusive 
lava  flows  a  small  amount  of  baking  or  hardening  of  the 
rocks  and  soils  on  which  they  rest  is  often  seen. 


186 


ROCKS  AND  ROCK  MINERALS 


Position  of  the  Rocks.  It  is  a  common  thing  to  observe 
that  the  width  of  the  contact  zone  varies  considerably 
from  place  to  place  about  the  intrusive  mass.  This  may  be 
due  to  underground  irregularities  in  the  igneous  rock  body, 
a  wide  extension  of  the  zone  pointing  to  a  corresponding 

extension  of  the  mass  be- 
low, as  illustrated  in  Figs. 
7 1  and  72.  In  the  stratified 
rocks  the  position  or  atti- 
tude of  the  planes  of  strat- 
ification to  the  intrusive 
mass  is  also  important. 
Thus  in  Fig.  73  the  beds  at 
B  dipping  into  the  mass 
of  granite  C  tend  to  have 
their  bedding  planes  and  joints  opened  by  the  upward 
movement  of  the  magma,  and  their  position  is  such  as  to 
facilitate  the  entrance  and  wide  extension  of  the  vapors 


Fig.  71.    Ground  Plan  or  Map  of  an 
Intruded  Stock  and  its  Contact  Zone. 


Fig.  73.    Vertical  Section  along 
Line  A— B  in  Fig.  71. 


Fig.  73.  Section  showing  width 

of  Contact  Zone  depending 

on  Position  of  Beds. 


and  heat,  thus  producing  a  broad  contact  zone.  On  the 
side  A  on  the  contrary  the  conditions  are  just  the  reverse 
of  this  and  a  much  narrower  contact  zone  is  the  result. 

Effect  on  Different  Kinds  of  Rocks.  In  a  general  way 
the  most  notable  effects  are  produced  on  sedimentary 
rocks  and  these  for  purposes  of  consideration  may  be 
divided  into  the  sandstones,  limestones,  clay  shales  or 
slates  and  their  various  admixtures.  On  pure  quartz 
sandstones  the  effect  is  relatively  slight  though  for  short 


GENERAL  PETROLOGY  OF  IGNEOUS  ROCKS      187 

distances  and  in  the  near  zone  of  most  intensive  action 
they  are  sometimes  found  hardened  into  quartzites. 
Pure  limestones  are  recrystallized  and  changed  into 
marble  and  not  infrequently  in  large  masses  and  extending 
over  considerable  distances.  The  most  notable  effects 
are  produced  when  the  limestones  are  impure,  containing 
quartz  sand  and  clay  mixed  with  them.  In  this  case  the 
Si02  drives  C02  out  and  carbonates  are  changed  to 
silicates.  If  the  limestone  is  a  dolomite  containing 
magnesia  the  results  are  more  complex.  Some  of  the 
simpler  of  these  changes  may  be  reaolily  shown  by  equa- 
tions which  represent  the  chemical  changes  involved. 

Calcite       Quartz      Wollastonite    Garb.  diox. 
CaCO3  +     SiO2       =  CaSiO3  +  CO3 

Dolomite        Quartz        Pyroxene 

CaMg(CO3)2  +  2  SiO2  =  CaMg(SiO3)2  +  2  CO8 

Calcite         Clay  Quartz  Garnet          Carb.  diox.  Water 

3  CaCO3  +  H4Al2Si2O9  +  SiO2  =  Ca3Al2Si3O12  +  3  CO2  +  2  H2O 

Calcite  Clay  Anorthite 

CaCO3  +  H4Al2Si2O,  =  CaAl2Si2O8  +  2  H2O  +  CO2 

In  some  cases  the  rock  is  thus  entirely  changed  into 
silicates  or  mixtures  of  them,  but  usually  it  consists  of 
impure  marble  or  altered  limestone  containing  the  min- 
erals aggregated  into  lumps  or  bunches.  Carbonaceous 
material  which  may  be  present  is  often  changed  into 
graphite.  In  addition  to  those  minerals  mentioned,  a 
variety  of  others,  whose  origin  depends  on  the  mineral- 
izing vapors  given  off  by  the  igneous  rock,  may  also  be 
formed,  such  as  mica  (phlogopite),  chondrodite,  horn- 
blende, vesuvianite,  epidote,  tourmaline,  etc.  In  these 
cases  the  main  materials  are  those  already  in  the  rock; 
the  vapors  furnish  the  volatile  components,  the  hydroxyl, 
boron,  fluorine,  etc.,  needed  for  their  composition.  Such 
minerals  furnish  transitions  to  the  more  typical  cases  of 
pneumatolytic  contacts  mentioned  below.  It  should  not 
be  forgotten  also  that  many  of  these  minerals  contain 


188  ROCKS  AND   ROCK  MINERALS 

oxide  of  iron,  ferrous  or  ferric  or  both,  and  this  must  come 
from  the  limonite  or  other  hydrated  iron  oxides  mixed  in 
with  the  impure  marly  beds  and  deposited  with  the  other 
material  at  the  time  of  their  formation.  Perhaps  of  the 
minerals  mentioned  garnets,  pyroxenes  and  vesuvianite 
may  be  taken  as  among  the  most  typical  of  such  occur- 
rences in  altered  limestones.  Many  instances  of  such 
contacts  are  known  in  various  parts  of  the  world  and  some 
of  them  have  become  famous  for  the  variety  and  beautiful 
crystallizations  of  the  minerals  which  they  afford  and 
which  are  to  be  found  in  all  mineral  cabinets. 

In  the  case  of  clay  shales  and  slates  variable  effects  are 
produced,  but  usually  ones  that  are  well  marked  and 
characteristic.  While  such  rocks  consist  mostly  of 
microscopic  fragments  of  quartz,  granules  of  clay,  mica, 
etc.,  there  is  considerable  variability  in  their  composition 
and  accordingly  a  difference  in  the  result  of  the  meta- 
morphism.  Sometimes  they  are  baked  into  a  dense,  hard 
rock  with  conchoidal  fracture,  of  a  black  or  very  dark 
stone  color,  called  hornstone,  which  closely  resembles  trap 
or  basalt.  Sometimes  they  are  like  the  hornstone  in 
hardness,  texture  and  fracture  but  differ  in  color,  being  of 
a  light  gray  to  green-gray  or  greenish  and  are  known  as 
adinole. 

In  other  cases  where  the  beds  are  more  rich  in  kaolin,  a 
mineral,  andalusite,  is  apt  to  develop  according  to  this 
formula: 

Kaolin      Andalusite  Quartz  Water 

H4Al2Si2O9  =  Al2SiO5        +         SiO2      +      2  H2O 

At  the  contact  a  rock  composed  largely  of  this,  often  in 
recognizable  grains  and  crystals,  and  mixed  with  a  brown 
biotite  in  glimmering  specks,  forms  a  granular  rock, 
generally  dark  in  color  and  much  resembling  an  igneous 
rock  in  texture.  All  visible  evidence  of  bedding  of 
sedimentary  character  is  lost.  This  would  be  termed  an 
andalusite  hornfels.  Further  from  the  contact  the  rock 
begins  to  lose  its  granular  texture;  it  becomes  more  schist- 


PLATE  12. 


' 


A.    INCLUSIONS   IN   GRANITE. 


B.   FRUCHTSCHIEFER. 


GENERAL  PETROLOGY  OF  IGNEOUS  ROCKS     189 

like  or  perhaps  slaty  and  is  dotted  with  the  andalusite 
prisms.  These  very  frequently  gather  up  the  dark 
organic  matter  of  the  rock  and  arrange  it  within  them- 
selves in  the  manner  characteristic  of  this  mineral.  They 
then  appear  dark  on  a  lighter  background,  as  seen  in 
Fig.  2,  Plate  12,  and  this  variety  of  rock  is  known  as 
"  Fruchtschiefer "  (fruitschist)  by  the  Germans.  The 
rock  has  much  the  character  of  a  fine-textured  mica- 
schist. 

Still  further  from  the  contact  the  effects  of  meta- 
morphism  are  less  and  less  marked,  the  beds  show  more 
and  more  of  their  original  sedimentary  nature;  in  this 
part  the  most  evident  effect  is  a  spotting  of  the  shales  or 
slates  from  collection  of  organic  matter  or  minerals  into 
more  or  less  well  defined  points  or  knots.  Such  a  develop- 
ment of  knots  is  one  of  the  most  characteristic  features 
of  moderate  contact  metamorphism  and,  when  encoun- 
tered in  the  field,  should  always  lead  to  search  for  more 
intensive  effects  and  the  possible  nearness  of  intrusive 
igneous  rock  bodies.  The  latter  may  of  course  be  below 
and  not  yet  exposed  by  erosion. 

Just  as  all  kinds  of  variations  between  sandstones, 
limestones  and  shales  are  found  in  nature,  so  do  the 
different  varieties  of  rocks  produced  by  contact  meta- 
morphism, as  described  above,  vary  and  grade  into  one 
another. 

In  the  nature  of  things  the  already  existent  igneous 
rocks  are  less  altered  by  the  contact  metamorphism  of 
following  intrusions  than  the  sedimentary  ones.  This  is 
particularly  true  of  the  granites  and  other  very  feld- 
spathic  rocks.  The  ferromagnesian  ones,  those  containing 
feldspars  rich  in  lime,  and  especially  those  composed 
chiefly  of  pyroxene,  show  at  times  considerable  effects. 
The  pyroxene  is  converted  into  hornblende  and  the  rock 
becomes  an  amphibolite  and  even  at  times  a  hornblende 
schist. 

Pneumatolytic  Contacts.     It  was  mentioned  above  in 


190  ROCKS  AND   ROCK  MINERALS 

connection  with  the  changes  observed  in  limestones  that 
minerals  appeared  whose  origin  was  due  to  the  mineraliz- 
ing vapors  given  off  from  the  igneous  mass.  At  times  in 
contact  zones  the  outer  rocks  may  be  converted  into 
masses  of  such  minerals,  testifying  to  the  abundance  and 
energetic  action  of  the  excluded  vapors.  Such  minerals 
as  tourmaline,  topaz,  fluorspar,  vesuvianite,  mica  (mus- 
covite),  etc.,  ones  containing  hydroxyl,  fluorine  and 
boron  are  characteristic  of  these  occurrences.  The 
masses  thus  formed  are  not  widespread  and  regular 
around  the  contact  but  appear  here  and  there,  espe- 
cially near  fissures,  sometimes  in  isolated  areas  in  the 
other  rocks,  sometimes  in  large,  sometimes  in  smaller 
lumps  and  masses,  following  the  irregular  escape  of 
the  gases. 

Contact  Zones  and  Ore  Bodies.  In  the  contact  zones  of 
igneous  rocks,  the  passage  of  the  vapors  and  the  move- 
ment of  heated  solutions  in  them,  combined  often  with 
their  own  chemical  composition,  which  causes  them  to 
react  with  the  solutions,  have  made  them  especially 
favorable  places  for  the  deposit  of  ores.  Their  loss  of 
volatile  substances  causes  a  reduction  of  volume,  they 
become  more  porous,  if  not  too  deeply  buried,  and  permit 
more  easily  the  circulation  of  fluids.  As  a  result  of  this 
we  find  many  valuable  deposits  of  the  ores  of  gold,  silver, 
lead,  copper,  etc.,  from  magmatic  waters,  in  such  contact 
zones.  In  places  in  the  mining  regions  of  the  Rocky 
Mountains  the  contact  between  sedimentary  beds  and 
intrusive  masses  of  granite,  porphyry,  etc.,  from  some 
elevated  point  may  be  followed  with  the  eye  for  miles  by 
the  successive  mines,  pits,  and  heaps  thrown  out  from 
prospects.  So  well  is  this  known  that  contacts  between 
porphyry  and  limestone  are  eagerly  sought  by  every  pros- 
pector. Any  adequate  treatment  of  this  subject  would 
carry  us  far  beyond  the  limits  of  this  work  and  further 
information  should  be  sought  in  those  treatises  which 
deal  with  the  origin  of  ore  deposits. 


GENERAL  PETROLOGY  OF  IGNEOUS  ROCKS      191 


Classification  of  Igneous  Rocks. 

Introductory.  There  is  probably  no  subject  in  the 
domain  of  natural  science  concerning  which  there  has  been 
and  is  to-day  less  agreement  than  in  the  classification  of 
igneous  rocks.  The  reason  for  this  is  that  there  are  no 
distinct  boundary  lines  drawn  by  nature  itself.  The 
igneous  rock  masses  of  the  earth  possess  certain  features 
which  may  be  used  to  distinguish  and  discriminate  them, 
one  from  another,  such  as  their  geologic  mode  of  occur- 
rence, their  mineral  composition,  their  texture,  and  their 
chemical  composition,  which  nearly  represents  that  of  the 
original  magma.  A  very  brief  inspection  serves  to  show, 
however,  that  in  each  of  these  features  gradations  exist 
without  hard  and  fast  lines.  If  we  classify  them  accord- 
ing to  mode  of  occurrence  and  divide  them  into  intrusive 
and  extrusive  rocks,  then,  for  example,  it  is  clear  that  every 
lava  flow  is  (or  was)  prolonged  into  depths  below  by  an 
intrusive  continuation  in  the  form  of  a  dike  or  volcanic 
neck.  We  should  have  to  separate  the  intrusive  from 
the  extrusive  at  some  point  by  an  arbitrary  plane;  above 
this  the  rock  would  receive  one  name,  below  it  another, 
though  it  is  clear  that  the  material  just  above  and  that 
just  below  would  be  absolutely  alike.  The  same  is  true 
when  we  consider  the  other  features  of  rocks  mentioned; 
they  are  found  to  grade  into  each  other  mineralogically, 
chemically  and  texturally,  and  where  lines  are  drawn  it 
must  be  done  arbitrarily.  It  is  due  to  these  facts  that 
so  much  diversity  of  opinion  has  existed  regarding  their 
classification,  some  laying  stress  on  one  feature,  some  on 
another.  By  general  common  consent  among  petro- 
graphers,  especially  since  the  use  of  the  microscope  has 
served  to  reveal  the  composition  of  dense  rocks,  a  large 
number  of  different  kinds  or  types  of  igneous  rocks  are 
recognized,  based  primarily  on  the  kinds  and  relative 
quantities  of  their  component  minerals  and  on  their 
texture,  but  as  to  the  manner  in  which  these  recognized 


192        ROCKS  AND  ROCK  MINERALS 

kinds  shall  be  grouped  in  a  classification  there  is,  as  stated 
above,  wide  diversity  of  opinion.  It  would  not  be  proper 
to  go  into  the  discussion  of  this  subject  further,  but  it 
should  be  clearly  understood  at  the  outset,  that  what- 
ever method  of  classification  of  igneous  rocks  is  used,  the 
boundary  lines  must  be  artificial  ones  and  in  many  cases 
just  where  a  rock  should  belong  must  be  a  matter  of 
opinion,  which  each  must  decide  for  himself. 

Older  Megascopic  Classification.  Before  the  micro- 
scope came  into  use  in  studying  rocks,  they  naturally 
divided  themselves  into  two  groups,  those  whose  com- 
ponent mineral  grains  were  large  enough  to  be  seen  and 
recognized  and  those  which  were  too  compact  to  permit 
this.  The  former  group  was  divided  into  different  kinds 
according  to  the  mineral  varieties  composing  them,  the 
latter  according  to  the  color,  texture,  luster  and  other 
physical  properties  they  presented.  In  this  manner  by 
common  usage  a  megascopic  classification,  extremely 
useful  for  geologic  and  common  purposes,  came  about, 
which  gave  rise  to  such  terms  as  granite,  diorite,  porphyry, 
greenstone,  basalt,  etc. 

Effect  of  the  Microscope.  When  the  microscope  came 
into  use  it  was  discovered  that  the  dense  rocks  could  be 
studied  and  their  component  mineral  grains  determined, 
nearly  as  easily  as  the  coarse-grained  ones,  and  the  result 
of  these  studies  showed  that  a  vastly  greater  diversity 
existed  among  them  than  had  been  suspected.  And 
among  the  coarser-grained  ones  it  was  also  found  that 
many  minerals,  until  then  not  known  in  them,  existed, 
and  that  great  variations  among  the  minerals  known  to 
compose  them  could  be  seen,  as  well  as  differences  in  tex- 
ture, etc.  To  express  these  differences  among  the  rocks 
and  to  connote  the  ideas  regarding  them  which  they 
engendered,  not  only  have  a  whole  host  of  new  rock  names 
arisen,  but  the  old  megascopic  terms  have  been  defined 
and  redefined  by  various  authorities,  until  they  have 
nearly  all  lost  their  original  significance. 


GENERAL  PETROLOGY  OF  IGNEOUS  ROCKS     193 

This  has  been  an  unfortunate  phase  of  the  history  of 
the  development  of  petrography  as  a  science,  because 
these  former  megascopic  or  field  names,  as  we  should 
term  them  now,  served  a  very  useful  and  necessary  pur- 
pose, which  the  more  exact  and  scientific  nomenclature 
of  modern  petrography  cannot  replace.  The  person  who 
desires  to  deal  with  rocks  and  name  them  from  the  mega- 
scopic, field  point  of  view,  such  as  the  field  geologist,  the 
engineer,  the  architect,  etc.,  is  left  without  any  equipment 
for  doing  so.  A  single  illustration  will  suffice.  The 
old  term  granite  meant  any  granular  igneous  rock  and 
then  later  one  composed  of  quartz  and  feldspar.  Now,  as 
used  by  modern  petrographers,  it  is  only  a  granite  in  case 
the  feldspar  is  chiefly  alkalic,  while  if  it  is  dominantly 
soda-lime  feldspar,  the  rock  is  termed  a  quartz  diorite, 
a  distinction  which  ordinarily  cannot  be  made  without 
microscopic  study. 

The  redefinition  and  specializing  of  these  useful  general 
field  terms  is  very  much  the  same  as  if  the  botanists  had 
redefined  such  terms  as  bush,  tree,  vine,  shrub,  etc.,  and 
had  made  them  the  names  of  particular  species  or  genera, 
so  that  if  tree,  for  instance,  were  properly  used,  it  would 
designate  only  oaks,  or  even  quercus  alba. 

In  the  meantime  in  the  world  at  large,  where  rocks  are 
commercially  dealt  with,  as  in  mining,  architecture,  etc., 
the  use  of  rock  names  in  the  old  way  has  gone  on 
quite  regardless  of  the  petrographers,  but  the  geologist 
or  engineer  who  has  endeavored  to  keep  up  with  the  de- 
velopment of  the  science  and  use  its  terms  megascopically 
has  carried  an  ever  increasing  load  until  finally  he  has 
been  compelled  to  become  a  petrographer  or  else  give  up 
in  large  part  any  independent  use  of  rock  names.  With- 
out doubt  it  is  largely  due  to  this  fact  that  every  advance 
in  the  definiteness  and  completeness  of  petrographic 
scientific  nomenclature  has  raised  a  wave  of  protest  among 
geologists. 

Present  Need  in   Classification.    It  is   clear  that   the 


194  ROCKS  AND  ROCK  MINERALS 

parting  of  the  ways  has  long  been  reached  and  it  ought 
to  be  definitely  recognized  that  the  further  development 
of  petrology  and  of  the  classification  and  nomenclature  of 
rocks  from  the  scientific  standpoint  must  be  left  largely 
to  petrographers,  while  those  who  have  occasion  to  deal 
with  them  in  the  purely  megascopic  manner  must  have  a 
method  of  classification  and  a  set  of  terms  of  a  totally 
different  scope  and  usage.  They  must  in  large  measure 
revert  to  that  which  was  in  vogue  before  the  microscope 
came  into  use. 

It  matters  little  whether  such  a  classification  is  com- 
pletely based  on  all  the  principles  underlying  scientific 
petrology  which  the  study  of  rocks  has  revealed  or  not;  to 
be  useful  it  must  be  practical  and  to  be  practical  it 
must  be  based  entirely  on  the  evident  megascopic  char- 
acters of  rocks,  such  as  can  be  seen  by  the  eye  or  pocket 
lens  or  be  determined  by  simple  means  at  every  one's 
command. 

Classification  used  in  this  Work.  As  it  is  the  object  of 
this  work  to  treat  rocks  from  this  point  of  view  the  follow- 
ing method  of  classification  has  been  adopted.*  First, 
the  rocks  are  considered  according  to  their  texture  and 
from  this  it  will  be  found  that  they  divide  naturally  into 
three  classes,  grained  f,  dense,  and  glassy. 

A.  Grained  Rocks.  By  this  is  meant  those  rocks 
whose  component  mineral  grains  are  large  and  distinct 

*  This  is  essentially  that  proposed  by  the  author  and  several 
other  petrographers.  "Quantitative  Classification  of  Igneous  Rocks," 
by  Cross,  Iddings,  Pirsson  and  Washington,  University  Chicago 
Press,  1903,  p.  180. 

t  The  term  "  grained  "  is  here  used  instead  of  "  granular  "  for 
two  reasons.  First,  because  granular  (from  granule  —  a  little 
grain),  strictly  speaking,  means  fine-grained,  while  the  rocks  included 
may  be  coarse,  medium  or  fine-grained.  Second,  because  granular 
is  used  by  many  petrographers  in  a  technical  way  as  an  equivalent 
to  "  even-granular "  and  opposed  to  porphyritic,  while  grained 
rocks  may  be  either.  Phanerocrystalline,  macrogranular,  mega- 
granular,  etc.,  have  much  the  same  meaning  but  it  is  better  to  use 
&  simple  English  word  than  a  compound  Latin  or  Greek  one. 


GENERAL  PETROLOGY  OF  IGNEOUS  ROCKS     195 

enough  to  be  seen  and  recognized  by  the  eye  alone  or 
with  the  lens.  No  hard  or  fast  line  can  be  here  drawn  as 
to  the  size  of  grain;  it  will  vary  with  the  kind  of  mineral, 
their  association  together,  and  on  the  experience  and  skill 
of  the  observer.  In  general  it  may  be  said  that  it  includes 
rocks  whose  average  size  of  grain  is  as  large  or  larger  than 
that  of  ordinary  loaf  sugar. 

B.  Dense  Rocks.     This  will   include   those  which   are 
nearly  or  wholly  of   stony  appearance   and  texture  but 
whose  minerals  cannot  be  determined,  because  the  con- 
stituent particles  are  too  minute.     They  may  even  appear 
homogeneous  but  are  generally  microcrystalline. 

C.  Glassy  Rocks.     Includes  those  wholly  or  in  part 
made  up  of  glass,  as  shown  by  their  vitreous  or  pitchy 
luster,    conchoidal   fracture,    and   other   characters    and 
appearance. 

Treatment  of  Porphyries.  Reference  is  had  in  the 
above  to  rocks  whose  average  size  of  grain  is  uniform  or 
nearly  so.  But,  as  explained  in  the  description  of  the 
porphyritic  texture  on  page  156,  many  igneous  rocks  are 
porphyries,  that  is,  they  contain  distinct  crystals  or 
phenocrysts  much  larger  in  size  than  that  of  the  average 
grain  of  the  groundmass  in  which  they  lie  embedded.  It 
is  assumed  that  in  general  the  size  of  the  phenocrysts  is 
such  that  they  can  be  distinctly  seen  and  the  particular 
kind  of  mineral  composing  them  can  be  recognized  or 
approximately  determined.  In  classifying  porphyries 
they  at  once  naturally  fall  into  two  classes;  first,  (D)  one 
in  which  not  only  the  phenocrysts,  but  also  the  mineral 
grains  of  the  groundmass  are  large  enough  to  be  determined 
and  second,  another  (E)  in  which  the  groundmass  is 
either  too  dense  to  be  made  out  or  (F)  glassy.  In  the 
former  case  (E),  two  subdivisions  can  be  made,  one  (a)  in 
which  the  phenocrysts  are  very  abundant  and  a  good  idea 
of  the  mineral  composition  of  the  rock  as  a  whole  may  be 
had  and  another  (&)  in  which  the  amount  of  groundmass 
is  predominant  and  this  cannot  be  done. 


196  ROCKS  AND   ROCK  MINERALS 

For  a  clearer  understanding  these  divisions  of  porphyries 
may  be  shown  in  tabulated  form. 

D.  Groundmass  grained,  recognizable. 

E.  Groundmass  dense,  unrecognizable. 

a.   Phenocrysts  very  abundant  and  recognisable. 
6.   Phenocrysts    not    very    abundant    or    rare, 
groundmass  very  abundant. 

F.  Groundmass  glassy. 

It  would  be  logical  to  make  the  same  subdivisions  a  and  6  under 
class  F  as  are  made  in  E  but  cases  where  glassy  groundmasses  are 
filled  with  abundant  recognizable  phenocrysts  dominating  in  amount 
over  the  groundmass  though  known,  are  not  sufficiently  common  to 
make  worth  while  the  subdivision  for  practical  purposes. 

No  sharp  lines  can  be  drawn  between  these  divisions; 
they  pass  into  one  another  gradually,  except  as  to  whether 
the  rock  is  glassy  or  stony  in  texture. 

It  is  to  be  observed  that  since  the  rocks  belonging  in 
division  D  have  their  mineral  constituents  determinable 
they  belong  in  the  same  category  as  those  in  A  of  the 
evenly  granular  ones  previously  mentioned,  so  far  as  this 
particular  is  concerned.  Likewise  in  division  E,  sub- 
division a,  if  half  or  'more  of  the  rock  is  composed  of 
recognizable  phenocrysts  its  general  mineral  character 
can  be  determined  and  it  falls  in  the  same  category.  But 
in  the  remaining  divisions  not  enough  of  the  mineral 
characters  can  usually  be  told  to  safely  identify  the  rocks 
on  this  basis  and  such  rocks  are  evidently  to  be  classed 
with  division  B,  dense  rocks  and  C,  glasses  which  are 
not  porphyries  and  which  cannot  be  subdivided  according 
to  mineral  composition. 

Subdivisions  of  Class  A.  The  igneous  rocks  having 
been  divided  into  classes  on  the  basis  of  texture  it  now 
remains  to  show  on  what  grounds  these  classes  can  be 
further  subdivided  and  the  individual  kinds  of  rocks, 
from  the  megascopic  standpoint,  obtained.  This  is  done, 
as  already  suggested,  in  the  even-  and  porphyritic-grained 


GENERAL  PETROLQGY  OF  IGNEOUS  ROCKS     197 

rocks  by  considering  their  mineral  composition.     First 
we  may  broadly  divide  them  into  two  main  groups. 

a.  Rocks  in  which  the  feldspars  or  feldspars  and 
quartz  predominate. 

6.  Rocks  in  which  the  ferromagnesian  minerals  (py- 
roxene, hornblende,  olivine,  etc.)  predominate. 

As  a  rule,  the  rocks  of  the  first  group  are  light-colored, 
white,  red  or  gray,  but  this  is  not  an  absolute  rule  since 
the  feldspars  are  sometimes  very  dark  from  an  included 
pigment.  In  general  the  rocks  of  the  second  group  are 
dark  in  color  to  black  but  this  is  also  not  an  invariable 
rule  since  some,  like  those  composed  wholly  of  olivine,  are 
rather  light. 

The  first  group  a  may  be  further  subdivided  on  the 
basis  of  the  relation  of  quartz  to  the  feldspars.  Those 
which  contain  an  appreciable  amount  of  quartz  with  the 
feldspars  fall  in  one  division  and  are  termed  granite,  when 
even  granular  in  texture,  and  granite  porphyry,  when  of 
porphyritic  texture,  while  those  in  which  quartz  is  absent 
or  is  present  in  inappreciable  quantity  are  called  syenite 
and  syenite  porphyry  respectively.  Further  division  of 
these  into  varieties  on  the  basis  of  particular  mineral 
characters  will  be  considered  in  the  description  of  them 
in  the  succeeding  chapter. 

The  second  group  6  is  subdivided  on  the  basis  of  the 
relation  of  the  feldspars  to  the  ferromagnesian  minerals, 
into  those  which  contain  feldspar,  subordinate  in  amount 
to  the  ferromagnesian  minerals,  and  those  in  which  it  is 
wanting.  Thus  we  have  as  follows: 

c.  Rocks  with  predominant  ferromagnesian  minerals. 

feldspar  subordinate. 

d.  Rocks  consisting  wholly  of  ferromagnesian  minerals. 

Group  c  is  subdivided  according  to  the  nature  of  the 
predominant  ferromagnesian  mineral  present.  For 
practical  purposes  there  need  be  only  two  considered 


198  ROCKS  AND   ROCK  MINERALS 

here;  if  it  is  hornblende  the  rock  is  diorite,  if  it  is  pyroxene 
it  is  gabbro.  The  means  for  distinguishing  between  horn- 
blende and  pyroxene  are  discussed  in  the  description  of 
these  minerals  in  a  preceding  part  of  this  work.  In  many 
cases,  especially  in  the  finer-grained  rocks  of  this  group, 
it  may  not  be  possible  to  distinguish  between  hornblende 
and  pyroxene  and  the  rock  may  then  be  termed  dolerite. 
This  name  would  then  mean  that  the  rock  consisted  chiefly 
of  indeterminable  predominant  ferromagnesian  minerals 
with  subordinate  feldspar.* 

Porphyries  occur  in  this  group  but  they  are  relatively 
of  less  importance  than  in  the  preceding  ones;  they  are 
treated  in  the  descriptive  part.  Rocks  in  which  ferro- 
magnesian minerals,  other  than  hornblende  and  pyroxene, 
predominate  over  feldspar  are  known  but  are  of  little 
practical  importance  in  a  megascopic  scheme  of  this 
character  and  are  therefore  omitted.  They  will  be 
mentioned  later. 

The  last  group  d,  consisting  wholly  of  ferromagnesian 
minerals,  is  divided  according  to  the  kinds  of  these  minerals 
present.  The  most  common  and  prominent  mineral  in 
the  group  is  pyroxene  but  this  is  usually  associated  with 
olivine  and  the  rock  is  termed  peridotite.  This  is  the 
most  common  member  and  may  be  used  as  a  general  term 
for  the  group.  If  olivine  is  absent  and  the  rock  consists 
wholly  of  pyroxene  it  is  pyroxenite,  if  of  hornblende, 
hornblendite.  Varieties  are  described  under  peridotite. 

*  In  this  usage  of  dolerite  the  author  adopts  and  follows  that 
proposed  by  Chamberlain  and  Salisbury  (Geology,  vol.  1,  p.  431, 
1904)  which  is  already  obtaining  considerable  vogue  and  from  such 
authority  is  likely  to  become  general.  In  Germany  the  term  is 
restricted  to  certain  coarse-grained  basaltic  rocks;  in  England  it 
has  had  a  certain  use  for  all  coarse-grained  basalts  and  for  rocks 
termed  elsewhere  diabases;  in  America  it  has  been  little  employed 
and  may  well  be  revived  as  a  field  name  in  the  sense  suggested. 
With  this  meaning  it  is  a  very  useful  term.  The  word  is  from  the 
Greek,  meaning  deceptive,  with  the  idea  that  the  pyroxene  cannot 
be  distinguished  from  the  hornblende. 


GENERAL  PETROLOGY  OF  IGNEOUS  ROCKS     199 

Porphyries  in  this  group  rarely  occur  and  are  of  no  prac- 
tical importance. 

In  summation,  in  considering  the  classification  of  the 
group  of  grained  rocks  whose  constituents  are  determinate, 
one  should  consult  what  has  been  said  regarding  the 
chemical  composition  of  igneous  magmas  and  the 
variations  in  mineral  composition  beginning  on  page  141. 
It  is  not  possible,  however,  to  classify  them  entirely,  for 
megascopic  purposes,  by  the  diagram  given  on  page  145, 
for,  in  general,  we  cannot  discriminate  between  the  different 
kinds  of  feldspars.  Thus  the  rock  there  shown  as  quartz 
diorite  must  be  classified  under  the  head  of  granite,  while, 
as  compared  with  the  diagram,  the  diorite  and  gabbro 
mentioned  above  broadly  overlap.  Still,  in  a  general  way, 
bearing  these  exceptions  in  mind,  the  classification,  dis- 
tinguishing between  the  feldspathic  and  the  ferromagne- 
sian  rocks,  brings  out  the  ideas  there  expressed.  The  rocks 
of  this  class  are  nearly  always  intrusive,  rarely  extrusive. 

Subdivisions  of  Class  B.  In  considering  the  second 
class  of  rocks,  B,  whose  texture  is  so  fine  or  dense  that  the 
mineral  grains  cannot  be  determined,  we  have  little  with 
which  to  classify  them  for  field  purposes  except  the 
color.  They  are  thus  divided  into  two  groups,  the  light- 
colored  and  the  dark-colored.  Of  course  if  the  rock  is  either 
white  or  black  there  can  be  no  difficulty  in  assigning  it  to 
one  or  the  other  of  these  two  divisions,  but  all  gradations 
of  color  exist  and  it  is  often  a  matter  of  pure  choice  to 
which  a  particular  rock  should  belong.  Evidently  some 
closer  definition  of  the  terms  is  needed.  We  may  do  this 
as  follows.  The  term  dark  includes  rocks  that  are  very  dark 
gray,  very  dark  green  or  black;  all  other  colors,  white,  red, 
purple,  yellow,  brown,  light  and  medium  gray,  light  and 
medium  green  are  light.  The  latter  are  known  under  the 
name  of  felsite,  while  the  former  or  dark  rocks  are  basalt. 
The  division  thus  made  also  expresses  in  a  general  way  an 
important  fact  concerning  their  composition,  for  the 
former  are  derived  from  magmas,  which,  under  different. 


200  ROCKS  AND   ROCK  MINERALS 

physical  conditions  producing  coarser-grained  rocks, 
would  crystallize  as  granites  and  syenites.  On  the  other 
hand,  the  basalts  represent  the  diorites,  gabbros  and 
peridotites  in  dense  or  fine-textured  forms.  While  many 
exceptions  will  be  found,  this  general  rule  holds  true  and 
the  light  rocks  as  defined  above  are  chiefly  feldspathic, 
the  dark  are  mainly  ferromagnesian. 

While  the  rocks  of  this  group  are  often  of  homogeneous 
texture  and  aspect,  they  are  also  very  often  porphyritic. 
If  the  amount  or  bulk  of  phenocrysts  in  relation  to  the 
fine  or  dense  (aphanitic)  groundmass  is  very  large,  say 
half  the  mass  of  the  rock  or  more,  such  porphyries  pass 
back  into  class  A,  of  grained  rocks  as  previously  explained. 
But  if  the  amount  of  phenocrysts  is  less  to  much  less  than 
the  groundmass  then  we  have  felsite  porphyry  and  basalt 
porphyry  respectively,  according  to  the  color  of  the  ground- 
mass.  It  has  also  been  suggested  that  they  may  be 
called  leucophyre  (light-colored  porphyry)  and  melaphyre 
(dark-colored  porphyry),  respectively.*  Further  sub- 
divisions of  these  porphyries  can  be  made  according  to 
mineral  character  of  the  prominent  phenocrysts.  Thus 
we  might  have  quartz- felsite-porphyry;  feldspar- felsite- 
porphyry;  hornblende-felsite-porphyry  or  quartz-,  feldspar- 
and  hornblende-leucophyre,  and  similarly  we  have  augite- 
basalt-porphyry,  mica-basalt-porphyry,  feldspar-basalt-por- 
phyry or  augite-,  mica-  and  feldspar-melaphyre.  Many 
combinations  of  this  kind  can  be  made  but  the  above  will 
suffice  as  examples.  The  rocks  of  this  class  are  some- 
times intrusive,  sometimes  extrusive. 

Subdivisions  of  Class  C.  The  rocks  of  the  third  class, 
C,  those  wholly  or  partly  of  glass,  are  distinguished  by 
their  glassy  or  resinous  luster  and  want  of  stony  texture. 
They  may  be  classified  as  follows: 

OBSIDIAN,  luster  strong,  bright,  glassy;  color  usually 
black,  sometimes  red,  more  rarely  brown  or 
greenish. 

*  Quantitative  Classification  of  Igneous  Rocks,  p.  184. 


GENERAL  PETROLOGY  OF  IGNEOUS  ROCKS      201 

PITCHSTONE,    luster    resinous    or   pitch-like;    colors 

various,  as  above,  but  black  less  common. 
PERLITE,  glassy  rock  with  perlitic  structure,  produced 

by  small  spheroidal  fractures;  usually  gray  in 

color. 
PUMICE,  highly  vesicular  glass  (see  page  158),  usually 

white  or  very  light-colored. 

Any  of  these  may  be  porphyritic  or  not;  though  cases 
of  porphyritic  pumice  are  much  less  common  than  in  the 
other  three.  When  porphyritic  a  general  name  for  them 
is  vitrophyre  (glass  porphyry)  and  different  varieties  may 
be  distinguished,  as  in  the  porphyries  of  the  class  above, 
according  to  the  kind  of  predominant  phenocrysts;  thus 
quartz-vitrophyre,  feldspar-vitrophyre,  etc.  The  rocks  of  this 
class  are  practically  wholly  confined  to  extrusive  lavas. 

Class  D.  In  addition  to  the  three  main  classes  of 
igneous  rocks  described  above  we  may  add  as  an  appendix 
in  a  fourth  class,  D,  the  fragmental  material  thrown  out 
in  volcanic  eruptions  and  already  mentioned  on  page 
140  as  tuffs  and  breccias. 

Such  material  serves  as  a  connecting  link  between  the  sedimentary 
and  igneous  rocks.  For,  as  it  falls  through  the  air,  it  becomes  assorted 
as  to  size,  and  successive  outbursts  thus  produce  rough  but  distinct 
bedding.  Or  it  may  fall  into  water  and  become  perfectly  stratified. 
Falling  on  the  land  it  may  cover  vegetation  and  contain  fossil 
imprints  of  plants,  leaves,  etc. ;  or  if  into  water,  of  marine  organisms. 
Thus  if  we  classify  volcanic  ash  beds  as  igneous  rocks  we  cannot 
say  that  a  distinguishing  feature  of  igneous  rocks  is  that  they  never 
contain  fossils.  See  remarks  on  page  133. 

Classification  Tabulated.  The  classification  which  has 
been  adopted  and  described  in  the  foregoing  may  now  be 
shown,  for  convenience  of  reference,  in  tabulated  form  on 
the  following  page. 

Classifications  based  on  Microscopic  Research.  In  the 
classification  previously  described,  the  color  and  texture 
of  rocks  play  a  prominent  part,  and  mineral  composition 
can  be  used  only  in  an  approximate  manner.  But  where 


202 


ROCKS  AND  ROCK  MINERALS 


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GENERAL  PETROLOGY  OF  IGNEOUS  ROCKS     203 

rocks  are  studied  in  thin  section  under  the  microscope 
texture  becomes  of  much  less  importance;  all  of  the 
minerals  and  their  exact  characters  can  be  discovered  and 
their  relative  proportions  made  out.  In  this  more  exact 
work  the  kinds  of  rocks  that  are  recognized  by  petrog- 
raphers  are  based  primarily  on  the  kinds  and  to  some 
extent  the  relative  proportions  of  the  component  minerals. 
This  makes  a  great  number  of  kinds  of  rocks  which  have 
been  named.  Generally  they  are  grouped  first,  according 
to  minerals  and  second,  according  to  texture;  some  petrog- 
raphers  lay  weight  also  on  their  mode  of  occurrence, 
whether  extrusive  or  intrusive,  while  others  add  to  this 
the  genetic  relations  or  groupings  which  they  show  in 
nature.  Classifications  have  also  been  proposed  in  which 
their  chemical  composition  plays  the  most  prominent 
part. 

Quantitative  Classification.  Recently  several  petrog- 
raphers,  including  the  author,  have  proposed  an  exact 
scientific  classification  of  igneous  rocks  based  on  their 
chemical  composition,  expressed,  however,  in  terms  of 
minerals  of  definite  composition,  called  standard  minerals. 
For  this  purpose  a  chemical  analysis  of  the  rock  is  neces- 
sary but,  where  this  cannot  be  obtained,  an  approximately 
correct  result  may  be  achieved  by  measurement  of  the 
minerals  under  the  microscope,  computing  from  this  their 
relative  bulk  and  weight,  and,  their  composition  being 
known,  reckoning  from  this  the  chemical  composition  of 
the  rock  as  a  whole,  as  if  obtained  by  chemical  analysis. 

The  chemical  composition  is  then  computed,  according 
to  a  set  plan,  into  the  relative  amounts  of  standard 
minerals.  These  standard  minerals  are  divided  into  two 
main  groups;  one  characterized  by  the  presence  of  alumina 
and  silica,  such  as  the  feldspars,  nephelite,  corundum  and 
quartz,  but  without  iron  or  magnesia,  the  second  charac- 
terized by  iron  and  magnesia  but  without  alumina,  such  as 
olivine,  diopside,  hypersthene,  aegirite  and  iron  ores.  The 
complex  ferromagnesian  minerals  which  contain  alumina, 


204  ROCKS  AND   ROCK  MINERALS 

such  as  hornblendes,  biotite,  augite,  etc.,  are  not  treated 
as  standard  minerals  because  it  is  better  to  consider  them 
as  compounds  of  simpler  molecules  of  the  two  preceding 
groups.  The  first  of  these  is  called  the  salic  (Si  and  Al) 
the  second  the  femic  (Fe  and  Mg)  groups  of  standard 
minerals  and  the  composition  of  the  rock  computed  in 
quantities  of  them  is  called  its  norm,  which  may  thus, 
when  hornblende  or  biotite  are  really  present  in  it,  differ 
considerably  from  its  actual  mineral  composition  or  mode. 
All  igneous  rocks  may  be  expressed  in  salic  and  femic 
minerals  and  according  to  the  relative  amount  of  each 
group  as  compared  with  the  other  they  are  divided  into 
five  classes,  persalane,  nearly  or  entirely  composed  of  salic 
minerals  (sal:  fern  >  7:  1);  dosalane,  mostly  salic 

(sal:  fern  <  7:1  >  5:3); 

salfemane,  equal  or  nearly  equal  quantities  of  each 
(sal:  fern  <  5:3  >  3:5); 

dofemane,  mostly  femic  minerals  (sal:  fern  <  3:5  >  1:7); 
and  lastly  perfemane,  nearly  or  entirely  femic 

(sal:  fern  <  1:7). 

Up  to  this  point  it  is  possible  to  use  this  classification  in  a 
megascopic  manner.  The  classes  thus  obtained  are  sub- 
divided into  orders  on  the  relations  of  the  salic  minerals, 
quartz,  feldspars  and  feldspathoids  (generally  nephelite), 
to  one  another  in  the  first  three  classes  and  on  somewhat 
similar  relations  among  the  femic  minerals  in  the  last  two. 
More  minute  consideration  of  the  mineral  oxides  divides 
the  orders  into  rangs  and  the  rangs  into  grads.  The 
proportions  by  which  they  are  thus  divided  are  always  the 
same  as  that  by  which  classes  are  made. 

Further  details  regarding  this  and  other  systems  of 
classification  founded  upon  results  obtained  by  micro- 
scopical research  are  to  be  found  in  the  list  of  works 
mentioned  on  page  10. 


CHAPTER  VH. 
DESCRIPTION  OF  IGNEOUS  ROCKS. 

Grained  Igneous  Rocks. 

As  explained  under  the  section  on  classification  the 
grained  igneous  rocks  are  those  whose  mineral  grains  are 
approximately  of  equal  size  and  large  enough  to  be 
identified  with  eye  or  lens,  aided  when  necessary  by 
chemical  or  physical  tests.  Those  rocks  whose  grain  is 
too  fine  to  permit  this  will  be  found  under  the  heading  of 
the  dense  igneous  rocks.  The  porphyries,  the  major  part 
of  whose  constituent  minerals  can  be  distinguished,  are 
described  in  the  following  section. 

GRANITE. 

Composition.  Granites  are  granular  rocks  composed 
of  feldspars  and  quartz.  Sometimes  they  consist  of  these 
minerals  alone  but  generally  there  is  more  or  less  mica 
present  and  often  hornblende. 

The  feldspar  is  the  predominant  mineral  and  is  readily 
recognized  by  its  appearance  and  cleavage.  Sometimes 
only  one  kind  of  feldspar  is  present  but  generally  there 
are  two,  orthoclase  and  soda-lime  feldspar.  They  may 
sometimes  be  distinguished  by  their  colors;  if  one  feldspar 
is  flesh-colored  to  red  while  the  other  is  white,  gray  or 
yellow,  it  is  pretty  certain  that  the  first  is  orthoclase,  the 
second  soda-lime  feldspar  (plagioclase).  Close  inspection 
of  a  cleavage  surface  of  the  latter  with  a  lens  may  show 
the  twinning  striations  (see  page  38)  but  the  grains  are 
rarely  coarse  enough  to  permit  this.  Rocks  in  which  the 
amount  of  plagioclase  is  greater  than  the  orthoclase  are 
called  quartz  diorites  by  petrographers  and  are  placed  in  a 


206  ROCKS  AND  ROCK  MINERALS 

separate  family,  but  this  cannot  be  done  in  megascopic 
determination  and  they  are  all  here  classed  as  granites. 
They  have  also  been  called  granodiorites  in  the  western 
United  States. 

Quartz  normally  occurs  as  formless  material,  filling  the 
interstices  between  the  other  minerals  and  hence  without 
definite  shape.  The  normal  color  is  white  to  dark,  smoky 
gray;  sometimes  it  is  red  from  included  hematite,  more 
rarely  a  bluish  color.  In  the  finer-grained  granites  the 
color  is  usually  light  or  white,  especially  in  those  of  a 
sugar  granular  texture.  It  is  recognized  by  its  oily, 
greasy  luster  and  conchoidal  fracture.  In  porphyritic 
granites  it  sometimes  occurs  in  large  dihexahedral  crystals 
or  round  grains. 

The  mica  may  be  either  the  light  or  colorless  muscovite 
or  black  biotite  or  both  kinds  may  be  present.  Cases 
where  muscovite  is  the  only  mica  are  rare.  Hornblende 
occurs  in  black  to  dark  green  grains  or  prisms.  It  is 
sometimes  the  only  dark  mineral  but  is  more  usually 
accompanied  by  biotite.  These  are  the  chief  minerals, 
but  if  the  rock  is  fairly  coarse-grained,  close  inspection 
will  commonly  show  occasional  metallic-looking  specks 
or  grains  of  iron  ore.  Sometimes  other  minerals  may  be 
seen,  brassy  crystals  of  pyrite,  dark  red  grains  of  garnet, 
etc.,  but  these  are  occasional  and  are  not  of  importance  in 
determining  the  rock. 

Texture.  In  ordinary  normal  granite  the  texture  is  an 
even  granular  one  and  alike  in  all  directions  through  the 
rock.  From  this  type  insensible  gradations,  sometimes 
in  the  same  mass,  may  be  observed  into  the  texture  of 
gneiss  which  becomes  noticeable  through  the  linear 
arrangement  of  the  components,  especially  the  micas. 
Thus  the  rock  passes  into  granite  gneiss.  The  normal 
texture  is  shown  on  Plate  13.  In  other  cases  a  tendency 
may  be  noted  for  some  of  the  orthoclase  crystals  to  be 
larger  than  the  average  grain  and  of  more  distinct  crystal 
form.  In  this  way  the  rock  becomes  porphyritic  and 


PLATE  13. 


A.    COMMON    GRANITE. 


13.    PORP11YRITIC    GRANITE. 


DESCRIPTION  OF  IGNEOUS  ROCKS  207 

when  this  is  pronounced  it  is  the  porphyritic  granite 
described  below.  Often  the  dark  minerals  tend  to  group 
or  bunch  together  in  spots. 

Color.  The  general  color  of  the  rock  depends  largely 
on  that  of  the  feldspar  and  in  the  proportion  of  this  to  the 
dark  minerals.  Thus  the  color  shades  from  white  into 
gray  to  dark  gray,  resulting  from  the  mottling  by  the 
biotite,  etc.  Such  types  are  very  common  wherever 
granites  are  abundant,  as  in  New  England.  More  rarely 
the  quartz  and  feldspar  are  themselves  gray  to  dark  gray 
and  thus  determine  the  color.  An  example  of  this  is  the 
granite  of  Quincy,  Mass.,  largely  used  as  a  building  stone. 
Another  very  common  type  of  coloring  is  one  in  which  the 
rock  is  flesh-colored,  pink  to  red  and  even  deep  red.  Such 
red  granites  are  found  in  Maine,  Missouri,  Colorado, 
Scotland  and  other  localities  and  are  largely  quarried  and 
used  for  building. 

Varieties.  The  varieties  of  granite  depend  on  the 
relative  proportions  of  the  light  and  dark  minerals,  the 
color  and  the  texture.  The  relative  amount  of  the  biotite 
(or  hornblende)  to  the  quartz  and  feldspar  may  vary 
widely;  it  may  be  entirely  wanting  or  it  may  be  present 
in  large  amount  and  make  the  rock  quite  dark.  Such 
extreme  cases  are  less  common.  The  grain  may  become 
as  coarse  as  large  peas  or  even  larger.  These  variations 
combined  with  those  in  color  produce  distinct  types  of 
granite  which  have  often  received  local  names.  Some 
other  varieties  are  described  in  following  sections. 

Porphyritic  Granite.  As  mentioned  above  the  feldspar 
may  partly  occur  in  large  distinct  crystals  or  phenocrysts. 
Strictly  speaking  this  would  cause  the  rock  to  become  a 
granite  porphyry  but  where  the  groundmass  in  which  these 
lie  is  as  coarse  as  an  average  granite  it  is  the  custom  to 
speak  of  it  as  porphyritic  granite,  laying  stress  on  the 
character  of  the  groundmass  rather  than  on  the  por- 
phyritic quality.  The  feldspar  phenocrysts  are  of  ortho- 
elase  and  have  the  forms  shown  under  feldspar,  page  35. 


208 


ROCKS  AND  ROCK  MINERALS 


Reflection  of  light  from  the  cleavages  of  these  on  the  rock 
surface  often  shows  they  are  in  twin  halves,  due  to  Carls- 
bad twinning.  The  size  of  these  phenocrysts  is  some- 
times quite  large,  an  inch  long  and  broad  or  even  more. 
An  illustration  of  this  type  of  granite  is  seen  on  Plate  13. 
Such  rocks  occur  in  New  Hampshire  and  other  localities 
in  New  England,  in  Colorado,  in  the  Sierra  Nevada 
Mountains,  in  England  (Dartmoor  and  elsewhere),  in  the 
Black  Forest  region  and  other  places. 

Chemical  Composition.  The  mass  compositions  of  a 
few  selected  granites  are  shown  in  the  analyses  given  here 
to  illustrate  the  kind  of  magma  from  which  such  rocks 
have  crystallized. 

ANALYSES    OF   GRANITES. 


I 

II 

III 

IV 

V 

SiO2 

77,6 

74.4 

71.2 

68.0 

66.3 

A1203 

12.0 

13.1 

13.7 

17.2 

16.0 

Fe203 

0.6 

0.7 

1.7 

3.1 

1.8 

FeO 

0.9 

0.9 

1.0 

0.4 

1.9 

MgO 

trace 

0.4 

0.8 

1.2 

1.1 

CaO 

0.3 

1.3 

2.3 

2.9 

3.7 

Na?O 

3.8 

2.6 

3.6 

3.2 

4.1 

K2O 

5.0 

6.1 

3.8 

3.9 

3.5 

H20 

0.2 

0.3 

1.7 

0.5 

XyO* 

0.2 

0.4 

0.2 

0.9 

Total  . 

100.6 

100.2 

100.0 

99.9 

99.8 

*  XyO  =  small  quantities  or  traces  of  other  oxides. 

I,  Hornblende  Granite,  Rockport,  Mass.;  II,  Biotite  Granite, 
Crazy  Mountains,  Montana;  III,  Granite,  Conanicut  Island,  Rhode 
Island;  IV,  Granite,  Kirkcudbright,  Scotland;  V,  "  Granodiorite  or 
quartz  diorite,"  Mariposa  County,  California. 

The  large  percentages  of  silica,  alumina  and  alkalies 
explain  the  predominance  of  feldspars  and  quartz.  With 
the  increasing  lime  in  the  last  two,  the  alkalic  feldspars 
give  place  in  precedence  to  plagioclase;  the  increasing 


PLATE  14. 


A.   EROSION   OF   GRANITE    IN   THE   HIGH   ALPS. 
(After  Duparc.) 


B.   EROSION  OF   GRANITE   IN   OLD  AND  LOW  MOUNTAIN 

REGIONS,  STONE  MOUNTAIN,   GEORGIA. 

(Georgia  State  Geological  Survey.) 


DESCRIPTION  OF  IGNEOUS  ROCKS  209 

iron  and  magnesia  show  increasing  amounts  of  the  dark 
minerals;  coincidently  with  this  the  silica  falls;  the  amount 
of  free  quartz  is  less  and  such  rocks  approach  the  next 
class,  the  syenites. 

Physical  Properties.  The  specific  gravity  of  granites 
varies  with  the  kinds  and  relative  amounts  of  the  com- 
ponent minerals;  from  2.63-2.75  is  the  ordinary  range, 
those  containing  more  ferromagnesian  minerals  being  the 
heavier.  The  average  weight  of  a  cubic  foot  of  granite  is 
about  165  pounds.  Usually  the  porosity  of  such  granites 
as  are  quarried  for  building  purposes  is  very  small,  the 
percentage  of  water  absorbed,  compared  with  the  weight 
of  the  dry  rock,  being  about  0.15  of  one  percent.  Thus  a 
cubic  foot  of  average  granite  if  completely  saturated  would 
absorb  about  4  ounces  of  water.  The  strength  of  granites 
in  resistance  to  crushing  is  very  great  and  probably  far 
greater  than  any  load  they  would  be  called  upon  to  bear 
in  architectural  work.  A  series  of  Wisconsin  granites 
tested  by  Buckley  showed  crushing  strengths  varying  from 
15,000-40,000  pounds  per  square  inch;  some  of  these  were 
very  high,  and  from  15,000  to  20,000  is  perhaps  the  aver- 
age. As  the  pressure  at  the  base  of  the  Washington  Monu- 
ment is  342.4  pounds  per  square  inch,  it  will  be  seen  there 
is  an  ample  reserve  in  most  cases. 

Uses  of  Granite.  As  is  well  known,  on  account  of  its 
great  strength  and  durability,  granite  is  extensively  used 
for  architectural  purposes.  Its  pleasing  colors  and  the 
high  polish  it  takes  cause  it  to  be  employed  as  an  orna- 
mental stone  in  interior  work,  in  monuments,  etc.  In 
one  respect,  however,  many  granites  have  a  defect  which 
somewhat  impairs  their  value  for  use  in  buildings  in 
large  cities.  This  defect  is  that  they  do  not  resist  fire  well, 
but  crack,  scale  and  sometimes  crumble  under  great 
heat.  One  reason  for  this  is  that  the  quartz  grains  are 
very  commonly  filled  with  minute  bubbles  containing 
water  or  liquid  carbonic  acid  gas  (CO2)  or  both.*  They 
are  so  minute  that  they  are  often  only  to  be  detected  with 

*  The  different  rates  of  expansion  of  quartz  and  feldspar  are  another  cause. 


210  ROCKS  AND   ROCK  MINERALS 

high  powers  of  the  microscope  in  thin  sections  but  they 
may  absolutely  swarm  in  the  quartz  and  constitute  an 
appreciable  fraction  of  its  bulk.  They  represent  material 
taken  up  or  included  at  the  time  of  its  crystallization. 
Under  the  action  of  heat  the  pressure  on  these  sealed 
crystal  flasks  becomes  enormous;  each  quartz  grain 
becomes,  so  to  speak,  a  veritable  tiny  bomb  and  eventually 
it  must  crack  in  all  directions  and  crumble  and  thus  injure 
the  strength  and  resisting  capacity  of  the  stone.  Feld- 
spars practically  never  suffer  from  liquid  inclusions,  like 
quartz,  nor  do  the  other  ordinary  rock  minerals,  so  that 
rocks  like  syenite  or  diorite,  in  which  quartz  is  absent  or 
only  sparingly  present,  make  in  respect  to  resisting  fire 
much  better  stone  than  granite. 

Jointing  in  Granite.  Granite  tends  to  a  block  jointing 
on  a  large  scale  in  the  great  stocks.  There  generally 
tends- to  be  three  distinct  sets  of  joints,  two  of  which 
approximate  to  the  perpendicular,  the  third  to  the  hori- 
zontal. Sometimes  these  are  nearly  at  right  angles  pro- 
ducing cubes,  more  often  at  angles  which  make  rhomboidal 
blocks.  Sometimes  the  horizontal  one  is  most  pronounced 
and  the  mass  has  a  sheeted  or  layer-like  character  sug- 
gesting bedding.  In  dikes  the  joints  are  much  more 
numerous  and  the  mass  breaks  into  small  blocks,  plates, 
etc.  This  jointing  of  granite  is  a  matter  of  much  impor- 
tance in  work  of  excavation,  in  mining,  tunnelling, 
quarrying,  etc.,  in  facilitating  removal  of  material,  but  it 
also  explains  why  every  granite  mass  is  not  suited  to 
furnish  material  in  blocks  of  sufficient  size  .for  construc- 
tional purposes.  Quarries  like  those  in  Finland,  in  the 
so-called  Rapakiwi  granite,  from  which  the  base,  a 
cube  of  30  feet,  and  the  shaft,  100  feet  high  by  15  feet  in 
diameter,  of  the  Alexander  monument  in  St.  Petersburg 
were  taken  and  those  in  Egypt  from  which  the  great 
obelisks  were  cut  are  not  common.  Compare  Plate  10. 

Erosion  Forms  of  Granite.  The  jointing  of  granite 
largely  conditions  the  work  of  erosive  agencies  on  the 


A.   Craftsbury,  Vermont. 


B.    Kortfors,  Sweden. 


C.    Stockholm,  Sweden. 
ORBICULAR  GRANITES. 


DESCRIPTION  OF  IGNEOUS  ROCKS  211 

mass  but  the  topographic  forms  produced  also  depend 
greatly  on  the  severity  with  which  these  act.  In  the 
high  mountain  chains  and  wherever  they  are  very  ener- 
getic, spires,  needles  and  castle-like  forms  are  produced, 
but  in  the  lower  massive  and  older  ranges  and  where 
glaciation  has  been  pronounced  the  granite  stocks  form 
more  smoothly  modeled,  rounded  or  dome-shaped  masses 
with  gentle  slopes  and  broad  valleys,  such  as  are  seen  in 
the  hills  and  mountains  of  New  England  and  in  parts  of 
Great  Britain.  The  views  on  Plate  14  are  illustrative  of 
this. 

Orbicular  Granite.  It  sometimes  happens  that  the 
component  minerals  of  a  granite,  instead  of  being 
uniformly  distributed  in  grains  of  about  the  same  size,  are 
collected  in  some  spots  in  an  unusual  way  and  arranged  in 
ovoid  or  spherical  bodies.  Thus  in  a  granite  from  Crafts- 
bury,  Vermont,  called  "  pudding  granite,"  the  rock  is  full 
of  nodules,  varying  from  the  size  of  a  pea  to  that  of  a  nut, 
composed  almost  entirely  of  agglomerated  leaves  of  black 
mica,  as  seen  on  Plate  15.  More  commonly  the  bodies 
are  composed  of  several  minerals  and  consist  of  a  nucleus 
with  a  concentric  outer  shell  or  shells.  The  component 
minerals  are  the  same  as  those  in  the  main  body  of  the 
rock  but  their  proportions  differ  in  the  nucleus  and  in  the 
shells,  sometimes  consisting  mostly  or  entirely  of  salic 
minerals,  while  some  shells  consist  mostly  of  ferromag- 
nesian  ones.  Their  appearance  is  shown  on  Plate  15. 

The  bodies  are  round,  ovoid  and  often  lenticular  or 
spindle  shaped,  as  if  drawn  out.  It  was  formerly  thought 
that  they  represented  pebbles  and  were  a  proof  of  the 
metamorphic  origin  of  granite  from  conglomerates,  but 
the  arrangement  and  regular  internal  structure  of  the 
ovoids  precludes  such  an  idea  and  it  is  now  generally  held 
that  they  are  due  to  some  process  of  differentiation  or 
aggregation  of  material  in  the  magma  with  subsequent 
crystallization,  though  in  some  cases  it  is  thought  that 
they  may  represent  inclusions  of  other  rocks  which  have 


212  ROCKS  AND  ROCK  MINERALS 

been  melted  up  and  recrystallized.  Granites  of  this  kind 
are  called  orbicular  and  though  not  common  they  have 
been  described  from  Sweden,  Finland,  Corsica,  Canada 
and  Rhode  Island.  Similar  structures  have  also  been 
found  in  diorites  and  gabbros. 

Miarolitic  Structure.  The  older  and  deeper  seated 
granites  and  especially  those  which  have  been  subjected 
to  heavy  mountain  making  pressures  show  little  or  nothing 
of  the  miarolitic  structure  described  on  page  159.  The 
conditions  have  been  unfavorable  for  the  formation  of 
such  cavities  or  under  the  pressure  they  have  been  oblit- 
erated. In  other  occurrences  and  in  the  younger,  higher 
or  unsqueezed  granites  these  cavities  may  occur,  and  on 
their  drusy  surfaces  fine  crystallizations  of  the  minerals 
may  be  seen.  The  crystals  from  such  cavities  in  the 
granite  of  the  Pike's  Peak  region  in  Colorado,  from  the 
Mourne  Mountains  in  Ireland,  from  Baveno  on  the  Lago 
Maggiore  in  Northern  Italy  and  other  localities  are  well 
known  in  mineral  collections. 

Pegmatite  Dikes.  These  are  very  common  in  granites, 
so  much  so,  that  when  this  word  is  used  a  granite  pegmatite 
is  usually  understood  unless  the  rock  is  otherwise  specified. 
They  have  the  general  characters  described  on  page  175 
and  the  following  ones.  The  chief  minerals  are  quartz 
and  feldspar,  the  latter  being  mostly  orthoclase  or  the 
variety  of  it  called  microcline,  though  albite  also  occurs. 
The  quartz  and  feldspar  are  apt  to  be  intergrown  in  such 
a  manner  that  the  interstices  of  a  spongy  quartz  crystal 
are  filled  by  an  equally  spongy  feldspar  crystal,  the  two 
sponges  thus  mutually  filling  each  other's  interstices  and 
interclasping.  As  the  quartz  has  no  cleavage  while  the 
feldspar  has,  the  cleavage  through  the  intergrown  mass 
is  that  of  the  feldspar  and  upon  such  surfaces  the  quartz 
appears,  repeating  its  tendency  to  crystal  form  again  and 
again  and  thus  producing  figures  which  recall  the  script 
used  in  Arabic  writings.  This  arrangement  is  called 
graphic  granite  and  a  figure  of  it  is  seen  on  Plate  16.  It 


PLATE  16. 


«.      . 


GRAPHIC-GRANITE,    OR   PEGMATITE. 


DESCRIPTION  OF  IGNEOUS  ROCKS  213 

shows  that  these  two  minerals  have  crystallized  simul- 
taneously. The  minerals  occur  also  separately  and  often 
in  huge  crystals  so  that  such  dikes  are  mined  for  com- 
mercial purposes,  the  quartz  and  feldspar  being  used  in 
several  technical  processes  such  as  the  manufacture  of 
china,  porcelain  ware,  etc.  The  large  crystals  of  mus- 
covite  mica  which  occur  in  them  are  the  source  of  this 
material  as  used  in  stove  windows,  lamp  chimneys,  paper 
making,  etc.  In  addition  to  these  chief  minerals  a  great 
variety  of  accessory  ones  are  found,  some  of  the  more 
common  of  which  are  tourmaline,  garnet,  beryl,  and 
spodumene  among  the  silicates,  apatite,  triphylite,  and  a 
series  of  related  phosphates,  and  a  variety  of  kinds  con- 
taining rare  earths.  Some  of  these  minerals  like  the 
colored  tourmalines,  topaz,  beryl,  etc.,  are  valuable  for  the 
material  suitable  for  cutting  into  gems  which  they  afford; 
others  are  useful  as  sources  of  the  rarer  elements  used  in 
chemical  and  some  technical  processes  such  as  the  making 
of  Welsbach  mantles.  A  full  list  of  all  the  minerals  known 
to  occur  in  these  pegmatites  would  cover  a  large  proportion 
of  all  the  kinds  known  in  mineralogical  science. 

Inclusions.  Schlieren.  It  is  not  uncommon  to  find 
in  granite  the  various  kinds  of  inclusions  described  on 
page  163  and  the  following.  Sometimes  the  composition 
and  form  of  these  show  that  they  are  fragments  of  pre- 
viously existing  rock  formations  broken  off  and  engulfed 
in  the  granite  magma.  These  are  most  common  near  the 
border  of  the  mass.  They  may  vary  in  size  from  an  inch 
across  or  less  to  masses  a  number  of  yards  long.  When 
they  are  found  in  the  center  of  the  mass  they  may  be 
suspected  of  having  sunk  into  it  from  the  former  overlying 
roof  of  other  rocks. 

In  other  cases  the  apparent  inclusions  are  the  schlieren 
described.  They  may  consist  wholly  or  nearly  so  of 
quartz  and  feldspar  and  thus  be  very  light  in  color  or 
extremely  rich  in  biotite  or  hornblende  or  both,  with 
iron  ore,  and  thus  be  very  dark  in  color.  Such  dark 


214  ROCKS  AND  ROCK  MINERALS 

streaks  may  at  times  be  due  to  melted  up  inclusions  but 
in  other  cases  they  may  be  caused  by  aggregations  of  the 
normal  dark  minerals  of  the  granite  and  in  general  are 
ascribed  to  processes  of  differentiation. 

Complementary  Dikes.  Very  frequently  it  will  be  found 
that  bodies  of  granite  are  cut  by  complementary  dikes 
as  described  on  page  167.  The  leucocratic  ones  are  com- 
monly composed  almost  solely  of  quartz  and  feldspar 
with  which  is  usually  associated  a  little  white  mica. 
The  rock  has  a  granular  appearance  and  this  variety  of 
granite  is  called  aplite.  Sometimes  small  black  specks 
of  biotite  or  hornblende  or  of  black  tourmaline  may  be 
seen  in  them  but  always  the  dark  minerals  play  a  very 
subordinate  role.  The  color  of  these  rocks  is  nearly  con- 
stantly very  light,  white,  flesh-color,  pale  yellow,  gray  or 
brown  being  common.  Sometimes  these  rocks  are  so 
fine  of  grain  that  they  pass  into  felsites  of  the  colors 
mentioned,  and  sometimes  they  are  porphyritic  with 
phenocrysts  of  quartz  or  feldspar  or  both  and  are  thus 
granite  or  felsite  porphyries.  But  most  commonly  they 
are  even-granular  with  a  grain  about  like  that  of  loaf 
sugar  and  the  dike  is  the  characteristic  mode  of  occurrence. 
They  are  mostly  noticed  cutting  the  granite  mass,  less 
commonly  the  surrounding  rocks.  They  are  of  all  sizes, 
from  a  fraction  of  an  inch  to  a  number  of  yards  in  breadth. 
If  the  larger  ones  are  traced  along  the  outcrop  it  may 
sometimes  be  found  that  they  change  into  pegmatite 
dikes. 

The  melanocratic  dikes,  sheets,  etc.,  complementary  to 
the  quartzo-feldspathic  aplites  described  above,  are  dark 
to  black  heavy  rocks  of  basaltic  aspect.  They  are  com- 
posed chiefly  of  biotite-mica,  hornblende,  pyroxene  and 
iron  ore  with  feldspars,  but  very  commonly  the  grain  is 
too  fine  for  these  minerals  to  be  distinguished  and  they 
are  to  be  classed  as  basalts,  or,  in  allusion  to  their  mode 
of  occurrence,  they  may  be  termed  lamprophyric  basalt. 
In  many  cases,  however,  when  biotite  is  the  prominent 


DESCRIPTION  OF  IGNEOUS  ROCKS  215 

ingredient  they  have  a  characteristic  glimmering  appear- 
ance or  the  plates  of  biotite  may  be  distinctly  seen,  and  in 
this  case  they  are  known  as  mica  traps.  The  most  char- 
acteristic color  of  these  rocks  is  a  dark  stone  gray.  Occa- 
sionally porphyritic  crystals  of  hornblende  or  of  feldspar, 
as  well  as  of  biotite,  appear  in  them  and  not  uncommonly 
fragments  of  the  granite  which  they  cut  and  of  other 
rocks.  They  also  at  times  contain  sulphurets  of  the 
heavy  metals,  usually  pyrite,  and  on  this  account  have 
been  prospected  or  mined  as  if  ore  veins,  generally  without 
much  result.  They  alter  and  weather  down  into  soft 
greenish  material  full  of  chlorite  or  into  brown  earthy 
masses.  The  earlier  stages  of  alteration  by  the  elements 
are  marked  by  the  formation  of  carbonates  and  they  then 
effervesce  freely  when  treated  with  acid. 

They  occur  characteristically  in  dikes,  usually  of  but  a 
few  feet  in  width,  but  as  previously  mentioned,  also  in 
intrusive  sheets,  small  laccoliths,  etc.  While  they  often 
cut  the  granite  they  are  more  apt  to  be  found  in  the  outer 
zone  of  rocks  surrounding  it  and  sporadic  occurrences  may 
be  discovered  a  number  of  miles  distant  from  the  parent 
mass.  The  origin  of  these  complementary  dikes  has  been 
already  discussed  on  pages  167  and  178. 

Contact  Phenomena.  It  is  especially  around  great 
granite  intrusions  that  the  contact  phenomena  described 
on  page  180  are  seen  in  their  greatest  development  and 
perfection.  In  the  endomorphic  form  the  granite  may 
become  a  felsite  or  granite  porphyry  at  the  contact,  or  it 
may  show  a  differentiated  border  zone  (see  page  165)  and 
become  so  enriched  in  the  dark  silicates  as  to  pass  into  a 
diorite  or  dolerite  border  facies,  or,  more  rarely,  on  the 
other  hand,  be  so  poor  in  these  as  to  present  a  marginal 
facies  of  aplite,  quite  like  that  seen  in  the  complementary 
dikes.  The  first  cases  mentioned  are  purely  textural 
modifications ;  the  second  are  chemical  and  mineralogical. 
More  rarely  cases  are  known  where  granites  have  a  border 
of  pegmatite.  With  respect  to  exomorphism  the  changes 


216  ROCKS  AND   ROCK  MINERALS 

described  in  the  previous  chapter,  on  account  of  the 
common  occurrence  of  granites,  are  more  frequently  seen 
and  have  been  more  extensively  studied  in  connection  with 
them  than  with  any  other  variety  of  igneous  rock.  Around 
the  great  granite  batholiths  these  effects  are  often  profound 
and  far  reaching,  involving  tracts  of  possibly  several  miles 
in  width.  Such  areas  are  often  of  great  interest  and  im- 
portance, not  only  from  the  geological  standpoint,  but 
because  they  are  frequently  the  site  of  important  ore 
deposits.  If  granite  comes  directly  against  sedimentary 
rocks  with  vertical  contact  and  the  latter  show  no 
evidence  of  metamorphism,  it  may  be  safely  assumed 
that  faulting  or  dislocation  has  brought  them  together. 

Weathering  of  Granite  into  Soil.  Through  the  action 
of  the  atmosphere,  of  water,  of  heat  and  cold,  granite 
breaks  down  into  soil.  In  northern  and  in  temperate 
regions,  such  as  eastern  North  America,  as  Merrill  has 
shown,  this  change  is  at  first  largely  a  mechanical  dis- 
integration and  the  resultant  material  differs  in  its  general 
chemical  composition  but  slightly  from  the  original  rock. 

In  appearance,  however,  as  it  changes  granite  may  alter 
considerably.  The  mica  tends  to  bleach  and  lighten,  and 
ferrous  compounds  tend  to  become  ferric  and  the  iron 
oxide  to  leach  out,  staining  the  rock  red  to  dark  brown. 
At  the  same  time  its  firm  texture  is  lost  and  it  becomes 
more  or  less  friable  and  crumbly.  Finally  it  falls  into  an 
angular  gravel  or  sand,  composed  mainly  of  particles  of 
quartz  and  feldspar,  called  gruss.  See  Plate  17. 

From  this  stage  as  the  change  into  soil  becomes  more 
complete,  the  most  important  process  is  the  conversion 
of  the  feldspar  into  kaolin,  according  to  the  following 
reaction. 

Orthoclase      Water  Carb.diox.         Clay  Quartz  Potas.  Carb. 

2KAlSi3O8  +  2  H2O  +  CO2  =  H4Al2Si2O9  +  4  SiO2  +  K2CO3 

This  reaction  begins  as  soon  as  the  rock  is  exposed;  it  has 
only  partially  taken  place  when  the  rock  crumbles,  but 


PLATE  17. 


GNEISSOID   GRANITE,    THROUGH    WEATHERING,   PASSING 
FROM   FIRM   ROCK   BELOW   INTO   ALTERED 

ROCK  AND   SOIL  ABOVE. 
(Merrill,  Bulletin  Geological  Society  of  America.) 


DESCRIPTION  OF  IGNEOUS  ROCKS  217 

after  that  goes  on  slowly  but  steadily  until  the  feldspar  is 
eventually  wholly  changed  into  clay.  As  a  result,  a  soil 
consisting  of  a  mixture  of  clay  and  quartz  sand,  stained 
reddish  or  yellowish  by  the  iron  compounds,  is  formed. 
Such  a  soil  is  called  a  loam.  Usually  the  process  is  not 
entirely  complete  and  the  soil  contains  more  or  less 
small  particles  of  feldspar  undergoing  alteration.  This 
has  an  important  bearing  on  the  self-renewal  of  its 
fertility. 

This  reaction  is  one  of  the  most  important  that  takes 
place  in  the  great  laboratory  of  Nature,  for  by  means  of  it, 
not  only  is  the  solid  rock  converted  into  soil,  but  one  of  the 
most  essential  of  plant  foods,  the  potash,  is  converted 
into  soluble  form  in  which  it  can  be  assimilated.  At  the 
same  time  the  other  essentials  of  plant  food,  the  silica, 
magnesia,  lime,  etc.,  are  also  unlocked  from  the  rocks  and 
rendered  available.  Thus  by  their  aid  plant  life  is  able 
to  grow  and  produce  from  water,  carbon,  dioxide,  etc., 
those  substances  upon  which  all  animal  life  ultimately 
depends. 

In  tropical  regions  the  decay  of  granite  gives  rise  to  a 
red  or  yellow-brown  ferrugineous  earth  to  which  the  name 
of  laterite  is  given.  It  has  been  shown  to  consist  of  a 
mixture  of  quartz  sand  with  hydrargillite,  a  clay-like  sub- 
stance with  the  composition  A1(OH)3,  colored  by  iron 
oxides.  But  the  name  has  also  been  applied  in  India  to 
soils  formed  from  the  basalts  of  the  great  Deccan  plateaux 
mentioned  later. 

In  tropical  deserts  the  surface  of  granites  becomes 
coated  by  a  brownish  or  black  skin,  sometimes  with  a 
luster  like  varnish,  due  to  the  alteration  of  the  iron- 
bearing  components  and  the  formation  of  iron  and  man- 
ganese compounds.  This  also  occurs  with  other  kinds  of 
rocks  as  well. 

Occurrence  of  Granites.  Granite  is  one  of  the  most 
common  and  widely  occurring  of  igneous  rocks,  and  plays 
a  prominent  role  in  the  formation  of  the  continental 


218  ROCKS  AND  ROCK  MINERALS 

masses.  In  the  form  of  great  stocks  and  batholiths  it 
forms  the  central  core  of  many  of  the  great  mountain 
ranges  and  is  revealed  by  later  erosion.  In  those  parts  of 
the  earth's  surface  which  have  been  subjected  to  repeated 
disturbances  of  the  crust  and  profound  erosion  granites 
are  common  rocks.  Thus  great  stocks  of  different  ages 
of  intrusion  are  found  in  eastern  Canada,  in  New  England 
and  generally  along  the  region  of  the  Piedmont  plateau 
from  southern  New  York  into  Georgia.  They  occur 
again  in  Missouri,  Wisconsin,  etc.,  in  isolated  areas,  but  in 
general,  until  the  Rocky  Mountains  region  is  approached, 
the  central  states,  which  compose  the  Mississippi  Valley, 
being  covered  with  stratified  rocks,  are  devoid  of  them, 
though  it  may  be  inferred  by  analogy  that  they  form  a 
large  part  of  the  basement  on  which  these  later  rocks  lie. 
In  the  Rocky  Mountains  and  in  the  far  western  states 
they  are  of  importance.  Likewise  in  Europe,  in  western 
and  southern  England,  in  Ireland  and  Scotland,  in  various 
places  in  France  and  Germany  and  in  the  Alps  they  are  of 
common  occurrence  and  their  exposures  form  considerable 
areas.  Such  a  list  of  occurrences  might  be  almost  in- 
definitely extended  but  enough  has  been  said  to  show 
their  importance  and  wide  distribution. 

SYENITE. 

Composition.  Syenites  are  granular  rocks  composed 
chiefly  of  feldspars.  They  differ  from  granites  in  that 
they  contain  no  quartz  or  only  a  negligible  quantity. 
They  may  consist  entirely  of  feldspar,  but  usually  more  or 
less  hornblende,  mica  or  pyroxene  is  present.  These 
however  are  subordinate  in  amount  to  the  feldspar,  since 
if  they  are  equal  to  or  exceed  it,  the  rock  becomes  a 
diorite.  If  the  rock  is  fairly  coarse-grained,  occasional 
particles  of  magnetite  and  other  minerals  may  be  seen, 
but  these  are  only  accessory  and  not  of  the  importance  of 
the  ones  mentioned.  Occasional  minerals  which  produce 
varieties  will  be  mentioned  presently. 


DESCRIPTION  OF  IGNEOUS  ROCKS  219 

In  strict  petrographic  classification  founded  on  microscopic 
examination  a  distinction  is  made  in  these  rocks  based  upon  the 
kind  of  feldspar.  If  the  latter  is  predominantly  an  alkalic  feldspar, 
without  lime,  the  rock  is  called  a  syenite,  as  above,  but  if  it  is  a  lime- 
soda  feldspar  or  plagioclase  the  rock  is  termed  a  diorite  without 
reference  to  the  quantity  of  dark  minerals  present.  This  dis- 
tinction, however,  cannot,  except  in  certain  exceptional  cases,  be 
carried  out  by  megascopic  examination  and  therefore  no  attempt  is 
made  to  separate  them  in  this  work. 

From  what  has  been  said  it  may  be  seen  that  the  mineral 
composition  of  syenite  may  vary  considerably;  there  may 
be  a  mixture  of  feldspars  present  or  only  one  kind,  either 
alkalic  or  lime-sodic,  and  there  may  be  variations  among 
the  ferromagnesian  minerals.  According  to  the  pre- 
dominant kind  of  the  latter,  the  rock  is  spoken  of  as  horn- 
blende syenite,  mica  syenite  or  augite  syenite.  All  of 
these  are  here  treated  under  the  general  heading  of  syenite, 
but  in  two  cases  the  rock  may  have  a  particular  mineral 
composition  which  makes  it  of  especial  interest  and  there- 
fore deserving  of  separate  description.  Generally  these 
two  varieties  may  be  identified  by  observing  with  care  the 
special  features  which  they  present  and  which  are  described 
beyond,  otherwise  they  cannot  be  distinguished  and  must 
be  classed  in  the  general  group  of  syenites.  These  two 
are  as  follows:  (A)  the  rock  contains  in  addition  to  the 
feldspars  and  other  minerals  a  notable  amount  of  nephelite 
or  this  and  its  congener  sodalite;  (B)  the  rock  consists 
entirely,  or  very  nearly  so,  of  soda-lime  feldspar  (labra- 
dorite).  We  may  then  divide  the  group  of  syenites  as 
follows : 

a.    Syenite,  in  general  or  common  syenite  consisting 

chiefly  of  feldspars,  without  quartz. 
6.     Nephelite    Syenite,    consisting    chiefly    of    alkalic 

feldspars  with  nephelite. 
c.     Anorthosite,  consisting  almost  wholly  of  labradorite. 

Properties  of  Syenite.  The  texture  of  syenites  is  usually 
even  granular,  but  sometimes  a  tendency  may  be  noticed 


220 


ROCKS  AND   ROCK  MINERALS 


for  the  feldspar  to  assume  a  flattened  tabular  form  like 
that  of  a  book,  its  cross  sections  on  the  rock  surface  are 
then  elongated  and  often  arranged  more  or  less  parallel, 
an  arrangement  which  is  thought  to  be  due  to  movements 
of  the  fluid  mass  during  crystallization.  This  variety  of 
texture  occurs  practically  only  when  the  feldspars  are  of 
the  alkalic  variety.  Porphyritic  varieties  also  occur  as  in 
granite  and  these  grade  into  syenite  porphyry.  The  color 
is  variable  like  that  of  granites;  white  to  pink  or  red,  or 
gray  or  yellow  tones  are  common,  gray  especially  so.  The 
specific  gravity  varies  with  the  minerals  and  their  propor- 
tions; it  may  extend  from  2.6-2.8.  In  a  tendency  to 
miarolitic  structure,  in  jointing,  in  erosion  forms,  in  altera- 
tion into  soil,  inclusions,  and  in  contact  metamorphism,  etc., 
what  has  been  said  in  regard  to  granite,  applies  also  to 
syenites  and  need  not  be  repeated.  They  are  also  accom- 
panied by  pegmatite  dikes,  but  these  are  not  so  common 
nor  so  well  known  as  the  granite  pegmatites.  They  also 
often  yield  a  great  variety  of  minerals. 

Chemical  Composition.  Chemically,  the  syenites  are 
distinguished  from  the  granites  by  a  lesser  amount  of 
silica,  which  accounts  for  the  absence  of  the  quartz;  in 
other  respects  they  resemble  them.  These  characters 
may  be  seen  in  the  following  table  of  analyses. 


ANALYSES    OF    SYENITES. 


SiO2 

A1203 

Fe203 

FeO 

MgO 

CaO 

Na2O 

K20 

H2O 

XyO 

Total. 

I 

60.7 

19.6 

1.5 

3.0 

0.8 

2.3 

4.9 

5.9 

0.3 

0.6 

99.6 

II 

60.2 

20.4 

1.7 

1.9 

1.0 

2.0 

6.3 

6.1 

0.3 

0.4 

100.3 

III 

61.6 

15.1 

2.0 

2.2 

3.7 

4.6 

4.3 

4.5 

0.7 

1.0 

99.7 

IV 

62.5 

16.5 

2.4 

2.0 

1.9 

4.2 

4.4 

4.6 

0.6 

1.3 

100.4 

I,  Belknap  Mountains,  New  Hampshire ;  II,  Fourche  Mountain, 
Arkansas;  III,  Little  Belt  Mountains,  Montana;  IV,  Plauen  by 
Dresden,  Germany.  Xyo  =  Small  amounts  of  other  oxides. 


DESCRIPTION  OF  IGNEOUS  ROCKS  221 

Occurrence  of  Syenites.  Syenites  are  not  very  common 
rocks,  and,  while  they  sometimes  occur  in  independent 
masses,  they  are  very  apt  to  be  connected  with  larger 
bodies  of  granite,  which  by  the  diminishing  of  quartz 
passes  into  syenite.  In  the  United  States  they  occur  in 
several  places  in  New  England,  at  Mount  Ascutney,  in  the 
White  Mountains  and  adjacent  region,  on  the  coast  north 
of  Boston,  also  in  Arkansas,  in  Montana  and  in  a  number 
of  other  localities.  They  are  found  in  several  places  in 
Germany  and  in  the  Alps.  An  important  area  of  them 
exists  in  South  Norway.  In  comparison  with  the  great 
batholiths  and  stocks  of  granite,  distributed  so  generally 
in  the  continental  masses,  they  are,  geologically  speaking, 
of  relatively  small  importance. 

Uses  of  Syenite.  For  all  constructional  and  other 
commercial  uses  syenite  has  the  same  value  as  granite. 
On  account  of  its  relative  rarity,  compared  with  the  latter 
rock,  it  is  however  little  used.  Its  crushing  strength  is 
equal  to  that  of  granite  and  from  experiments  by  J.  F. 
Williams  on  syenite  from  Arkansas  it  may  be  even  greater. 
Its  weight  per  cubic  foot  is  about  the  same.  The  absence 
of  the  quartz,  which  is  harder  than  feldspar,  should  make 
it  an  easier  stone  to  dress  and  polish,  and  practically  it 
resists  weathering  as  well,  if  not  better.  The  absence  of 
the  quartz  makes  it  also  a  better  stone  in  resisting  the  heat 
of  fires  (compare  granite,  page  209)  and  it  would  be  in 
consequence  a  more  advantageous  material  for  building 
in  our  large  cities.  If  these  advantages  over  granite 
were  more  generally  understood  it  is  probable  that  the 
accessible  occurrences  in  New  England  would  be  more 
extensively  exploited.  The  beautiful  dark  gray  syenite 
of  South  Norway  with  pearly  blue  reflections  is  con- 
siderably used  in  northern  Europe  as  an  ornamental 
stone. 

Nephelite  Syenite.  This  variety  is  distinguished  by  the 
fact  that  in  addition  to  the  feldspars,  which  are  almost 
wholly  alkalic  in  composition,  a  considerable  amount  of 


222  ROCKS  AND  ROCK  MINERALS 

nephelite  is  present.  This  mineral  is  sometimes  flesh- 
colored  but  usually  it  is  smoky  gray.  In  its  lack  of  good 
cleavage  and  oily,  greasy  luster  it  resembles  quartz,  but 
can  be  readily  distinguished  from  it  by  the  gelatinization 
test  (page  115).  It  is  generally  present  in  formless  grains 
mixed  with  the  feldspars,  but  sometimes  shows  the  outlines 
of  a  crystal  form.  It  is  apt  to  be  accompanied  by  sodalite, 
which  is  often  of  a  bright  blue  color,  in  grains  or,  if  the  rock 
is  very  coarse-grained,  in  lumps  and  masses;  if  it  is  thus 
present  it  is  useful  in  aiding  to  distinguish  this  rock  from 
common  syenite.  Nosean  and  cancrinite  may  also  be 
present.  Mica  (lepidomelane),  hornblende  (arfvedsonite) 
and  pyroxene  (aegirite)  are  usually  present  in  variable 
amounts,  in  plates,  grains  or  prisms,  of  a  black  color,  and 
containing  considerable  soda  and  iron.  The  presence  of 
soda  in  the  minerals  of  this  rock  is  readily  understood 
from  a  consideration  of  the  chemical  analyses,  given 
beyond,  which  show  the  composition  of  the  magma  from 
which  they  crystallized. 

The  color  of  nephelite  syenites  is  variable  but  commonly 
gray.  The  texture  is  granular,  sometimes  rather  por- 
phyritic.  The  book-shape  of  the  feldspars  mentioned 
above  is  common.  The  rock  is  liable  to  contain  many 
accessory  minerals  but  usually  only  in  microscopic  sizes; 
some  of  these  are  of  especial  interest  on  account  of  the 
rare  earths  they  contain.  It  is  also  prone  to  exhibit  in 
places  great  variations  of  the  constituent  minerals  giving 
rise  to  different  facies.  Many  of  these  varieties  have 
received  special  names.  Usually  it  is  cut  by  comple- 
mentary dikes  and  these  are  of  a  different  character  from 
those  found  associated  with  granites  and  common  syenites; 
one  is  a  pale  brown  or  pink  felsite,  another  a  bright  to 
dark  green  rock  called  tinguaite  which  owes  its  color  to 
microscopic  needles  of  aegirite ;  it  usually  contains  nephelite 
and  gelatinizes  with  acid.  The  complementary  lam- 
prophyres  to  these  are  heavy  dark  rocks  of  basaltic  aspect 
often  showing  distinct  to  large  phenocrysts  of  biotite, 


DESCRIPTION  OF  IGNEOUS  ROCKS 


223 


augite  or  hornblende;  they  are  particular  varieties  of 
melaphyre  (basalt-porphyry). 

The   chemical   composition  is   illustrated  in  the  two 
following  analyses  of  nephelite  syenites. 


Si02 

A1203 

Fe203 

FeO 

MgO 

CaO 

Na2O 

K20 

H2O 

XyO 

Total. 

I 

58.8 

22.5 

1.5 

1.0 

0.2 

0.7 

9.6 

4.9 

1.0 

0.3 

100.5 

II 

53.1 

21.2 

1.9 

2.0 

0.3 

3.3 

6.9 

8.4 

1.4 

2.0 

100.5 

I,  Salem  Neck,  Mass.;  II,  Magnet  Cove,  Arkansas.  XyO,  small 
quantities  of  various  oxides. 

It  will  be  seen  that  the  most  striking  thing  in  respect 
to  these  magmas  are  the  high  amounts  of  alumina  and 
alkalies,  with  moderate  silica.  It  is  this  which  causes  the 
formation  of  nephelite  (Na20  .  A12O3  .  2  SiO2)  rather 
than  albite  (Na2O  .  A1203  .  6  Si02),  there  being  not 
enough  silica  to  convert  all  the  alumina  and  alkalies  into 
feldspar.  From  this  it  may  be  seen  that  free  quartz  and 
nephelite  cannot  crystallize  from  the  same  magma;  the 
silica  would  convert  the  nephelite  into  albite,  and  therefore 
these  two  minerals  are  not  found  in  the  same  rock.  Some- 
times nosean  is  present;  cancrinite  may  also  occur,  and  of 
the  associated  minerals  zircon  is  perhaps  the  most  charac- 
teristic. In  this  connection  the  description  of  the  felds- 
pathoid  group  in  the  part  dealing  with  minerals  should 
be  read. 

Pegmatite  dikes  occur  in  connection  with  nephelite 
syenites  and  those  of  South  Norway  and  Greenland  are 
especially  interesting  for  the  great  variety  of  minerals, 
many  of  them  composed  in  part  of  the  rarer  elements, 
which  they  have  afforded. 

Nephelite  syenites  usually  occur  in  rather  small  stocks 
or  large  dikes;  relatively  large  areas  of  them  are  known  in 
only  a  few  places,  Greenland,  South  Norway  and  Lapland. 
They  are  of  rare  occurrence  and  geologically  are  of  small 


224  ROCKS  AND  ROCK  MINERALS 

importance  compared  with  granites,  gabbros  and  diorites. 
In  the  United  States  they  are  found  at  Litchfield,  Maine; 
Red  Hill,  New  Hampshire;  Salem,  Massachusetts;  Beem- 
erville,  New  Jersey;  Magnet  Cove,  Arkansas;  Cripple 
Creek,  Colorado;  in  western  Texas  and  a  few  other  local- 
ities. In  Canada,  at  Montreal;  Dungannon,  Ontario;  Ice 
River,  British  Columbia.  Noted  localities  for  these  rocks 
and  their  associated  minerals  in  Europe  are  in  South 
Norway;  Alno  Island,  Sweden,  Kola  Peninsula  and  Miask, 
Ural  Mountains,  Russia;  Foya,  Portugal  and  Ditro,  Tran- 
sylvania. The  rock  is  too  uncommon  to  be  of  commercial 
importance  but  makes  an  excellent  building  stone  where 
it  occurs.  That  in  the  neighborhood  of  Magnet  Cove, 
Arkansas,  has  been  thus  used. 

Anorthosite.  This  rock  is  composed  wholly,  or  nearly 
so,  of  a  soda-lime  feldspar,  usually  the  variety  described 
as  labradorite.  Sometimes  small  quantities  of  a  ferro- 
magnesian  mineral,  pyroxene,  is  sprinkled  through  it  in 
grains  and  specks,  or  small  masses  of  magnetite  or  some 
other  iron  ore  can  be  seen.  This  simple  mineral  com- 
position makes  it  resemble  in  the  hand  specimen,  especially 
when  the  grain  is  rather  fine  and  the  colors  light,  both 
marble  and  quartzite,  also  rocks  consisting  of  a  single 
mineral.  From  the  former  it  is  easily  told  by  its  superior 
hardness,  since  the  feldspar  cannot  be  scratched  by  the 
knife,  while  marble  is  easily  cut  or  scratched,  and  from  the 
latter  by  the  cleavage  of  the  crystal  grains  which  can 
usually  be  easily  seen  with  a  lens.  While  these  characters 
help  to  distinguish  the  rock,  its  identification  can  only  be 
made  certain  by  the  determination  of  the  kind  of  feldspar 
present ;  otherwise  it  can  merely  be  referred  to  the  general 
group  of  syenites.  This  can  only  be  done  in  the  field 
when  the  cleavage  surfaces  of  the  feldspars  are  sufficiently 
large  to  permit  one  to  see  on  them  the  characteristic 
twinning  striations  of  plagioclase  (see  page  38).  In 
the  laboratory  the  feldspar  can  be  identified  by  blowpipe 
and  chemical  tests. 


DESCRIPTION  OF  IGNEOUS  ROCKS  225 

The  color  of  the  rock  is  normally  white,  and  this  is 
sometimes  seen,  but  generally  it  is  colored  yellowish  to 
brown,  or,  more  commonly,  some  shade  of  gray,  light 
gray,  blue  or  smoky  to  dark  gray  and  almost  black. 
The  very  dark  exotic  color  is  due  to  an  included  pig- 
ment, perhaps  ilmenite  dust,  but  it  is  notable  that 
where  these  rocks  have  been  subjected  to  erogenic  pres- 
sure, and  especially  when  they  have  been  sheared  and 
granulated  and  have  assumed  gneissoid  structure,  the 
dark  color  tends  to  disappear  and  they  become  lighter. 
The  chemical  composition  is  practically  that  of  a  labra- 
dorite  feldspar  (Anal.  IV,  page  43). 

Mineralogically,  the  anorthosites  are  related  to  the 
gabbros,  for  they  contain  the  same  kind  of  feldspar  and 
often,  as  stated  above,  there  is  more  or  less  pyroxene;  if 
this  latter  increases  in  amount,  passages  into  gabbro  may 
occur;  and  in  gabbros,  phases  poor  in  pyroxene,  and  thus 
transitional  to  anorthosite,  are  found.  Geologically,  how- 
ever, they  occur  quite  independently  of  gabbros.  They 
are  not  common  rocks,  so  far  as  the  number  of  occurrences 
is  concerned,  but  are  of  importance  from  the  large  and 
sometimes  vast  masses  which  they  form,  notably  in 
Canada  and  Norway.  They  are  found  in  Canada  in 
separate  areas  from  the  west  coast  of  Newfoundland  and 
the  east  coast  of  Labrador  down  through  Quebec  into 
eastern  Ontario.  One  of  these  areas  drained  by  the 
Saguenay  River  covers  nearly  6000  square  miles  while  one 
near  Montreal  comprises  about  1000  square  miles. 

Another  region  is  in  the  Adirondack  Mountains  in 
northern  New  York  state,  which  is  in  large  part  com- 
posed of  this  rock.  Small  occurrences  of  a  nearly  related 
type  are  found  also  in  the  White  Mountains,  New  Hamp- 
shire. It  is  found  again  in  considerable  masses  in  Minne- 
sota on  the  Lake  Superior  coast. 

In  Europe,  anorthosite  occurs  in  Norway  in  large  areas 
on  the  west  coast  at  Bergen,  at  Ekersund  and  on  the 
Sognfiord.  It  is  also  found  in  Volhynia  in  Russia. 


226        ROCKS  AND  ROCK  MINERALS 

The  labradorite  of  this  rock  sometimes  shows  a  beautiful  opal- 
escent play  of  colors,  especially  a  deep  blue.  Cleavage  pieces  from 
the  coarse  and  massive  rock  of  the  coast  of  Labrador  have  long  been 
known  and  cut  as  ornamental  stones.  Similar  material  comes  from 
near  Zitomir  in  Volhynia. 

Corundum  Syenite.  In  all  the  different  varieties  of  syenite 
described  above,  instances  have  been  found  in  which  the  rock  con- 
tains, in  addition  to  the  usual  constituents,  a  notable  amount  of 
corundum.  The  appearance  of  this  mineral  is  due  to  the  fact  that 
the  magma  contains  more  alumina  than  the  alkalies  and  lime  present 
can  turn  into  feldspars  and  feldspathoids,  and  this  excess  is  forced 
to  crystallize  out  as  corundum  (A12O3),  just  as  in  granites  the  excess 
of  silica  is  compelled  to  form  quartz  (SiO2).  The  mineral  occurs  in 
crystals,  either  hexagonal  prisms  or  barrel-shaped,  or  in  grains  and 
lumps,  and  is  usually  of  a  gray  color.  It  is  easily  identified  by  its 
excessive  hardness. 

Such  occurrences  have  been  found  in  central  Montana  in  common 
syenite;  in  the  counties  of  Renfrew,  Hastings  and  others  in  Ontario, 
Canada,  in  syenite  and  nephelite  syenite;  these  rocks  have  been 
traced  in  a  belt  a  distance  of  over  a  hundred  miles;  it  occurs  in  a 
similar  manner  in  the  Ural  Mountains  and  in  Coimbatore  district, 
India.  Anorthosites  containing  corundum  are  known  from  Clay 
County,  North  Carolina;  Lanark  County,  Ontario,  and  from  the  Ural 
Mountains.  Some  of  these  occurrences,  notably  the  ones  in  Canada, 
are  of  economic  value  as  a  source  of  this  valuable  abrasive.  Corun- 
dum also  occurs  in  other  kinds  of  igneous  rocks  as  mentioned  under 
dunite. 

DIORITE. 

Composition.  The  diorites  are  granular  igneous  rocks 
composed  of  hornblende  and  feldspar  of  any  kind,  in 
which  the  amount  of  hornblende  equals  or  exceeds  the 
amount  of  feldspar.  Usually  more  or  less  iron  ore  in 
fine  grains  can  be  seen,  and  very  often  considerable  biotite 
is  present  in  shining  flakes,  with  sometimes  a  bronzy 
luster.  The  hornblende  is  usually  black,  sometimes 
dark  green,  and,  while  often  in  bladed  or  prismatic  forms, 
it  is  also  often  in  short  thick  crystals  or  grains  and  some- 
times in  small  masses  of  them  and  of  biotite  separated  by 
the  light-colored  feldspar.  For  its  recognition  and  dis- 
tinction from  pyroxene  see  page  66.  While  any  kind  of 
feldspar  may  be  present,  in  the  great  majority  of  cases, 


PLATE  18. 


A.    ORBICULAR   DIORITE, 
CORSICA. 


B.    DIORITE. 


C.   DIORITE,   COMMON   TYPE. 


DESCRIPTION  OF  IGNEOUS  ROCKS  227 

as  learned  from  microscopical  studies,  it  is  a  soda-lime 
variety,  containing  considerable  lime.  This  latter  point 
however  can  rarely  be  determined  on  the  hand  specimens 
because  the  rock  is  not  often  coarse  grained  enough  to 
permit  the  recognition  of  twinning  striations  on  their 
cleavage  surfaces.  It  is  not  uncommon  for  some  quartz 
to  be  present  and  this  can  sometimes  be  identified  with 
the  lens. 

While  the  rocks  determined  as  diorites  by  this  megascopic  classi- 
fication will  correspond  in  a  general  way  with  the  greater  part  of  the 
diorites  of  the  more  strict  classifications  founded  on  microscopic  and 
chemical  methods  they  also  include  some  less  common  rocks  which, 
for  one  reason  or  another,  have  been  given  various  names  by 
petrographers. 

General  Properties.  The  color  of  diorites  is  dark-gray 
or  greenish,  running  into  almost  black  in  some  varieties. 
It  results  from  the  color  of  the  hornblende  and  the  pro- 
portion of  this  to  feldspar.  The  different  varieties  are 
due  to  the  color,  coarseness  of  grain,  etc.  The  texture  of 
the  rock  is  the  granular  one.  The  porphyritic  texture, 
while  not  unknown,  is  far  less  common  than  in  granite. 
Sometimes  the  black  hornblende  prisms  are  distinct 
enough  to  produce  an  impression  of  porphyritic  texture 
which  is  dispelled  as  soon  as  one  compares  the  average 
size  of  the  crystal  grains.  Orbicular  structures  are  known 
to  occur.  A  rock  from  Corsica  exhibiting  it  has  been  used 
somewhat  as  an  ornamental  stone;  it  is  illustrated  on 
Plate  18.  Miarolitic  cavities  occur  as  in  granite;  they  are 
often  masked  by  being  filled  with  calcite.  Pegmatite 
dikes  also  occur  and  the  minerals  are  somewhat  different 
from  those  in  the  granites.  Fluidal  or  somewhat  parallel 
arrangements  of  the  component  minerals  are  not  uncom- 
monly seen,  and  these  produce  tendencies  to  gneissoid 
structure.  Diorites  are  also  frequently  cut  by  com- 
plementary dikes,  of  much  the  same  general  appearance 
as  those  in  granites,  or  these  are  found  in  their  immediate 
neighborhood  in  dikes  and  sheets.  Thus  they  may  be 


228 


ROCKS  AND  ROCK  MINERALS 


traversed  by  light-colored  aplites  and  felsites  and  by 
dark,  heavy,  basaltic  traps. 

Their  jointing  is  like  that  described  for  granites. 

Chemical  Composition.  This  varies  considerably  with 
the  relative  amounts  of  feldspar  and  hornblende,  with 
the  particular  varieties  of  these  two  which  are  present,  and 
is  also  somewhat  influenced  by  the  accessory  minerals 
which  may  occur.  The  following  table  illustrates  this 
and  it  shows  also  how  the  increase  of  lime,  iron  and 
magnesia  over  -the  proportions  of  these  oxides  in  granites 
and  syenites,  causes  the  increase  in  the  amount  of  horn- 
blende. 

ANALYSES    OF    DIORITES. 


SiO2 

A1203 

Fe203 

FeO 

MgO 

CaO 

Na2O 

K20 

H2O 

XyO 

Total. 

I 

55.1 

20.2 

1.5 

4.3 

1.8 

7.0 

4.3 

2.8 

1.1 

1.7 

99.8 

II 

58.0 

18.0 

2.5 

4.6 

3.6 

6.2 

3.6 

2.1 

0.9 

1.2 

100.7 

III 

47.1 

18.1 

3.0 

8.5 

7.3 

6.6 

2.4 

2.8 

3.6 

0.5 

99.9 

IV 

43.9 

16.2 

4.0 

10.1 

5.1 

9.6 

2.9 

1.5 

1.6 

4.9 

99.8 

I,  Diorite,  Little  Belt  Mountains,  Montana;  II,  Electric  Peak, 
Yellowstone  Park;  III,  Malvern  Hills,  England;  IV,  Belknap  Moun- 
tains, New  Hampshire.  XyO  =  Small  quantities  of  various  oxides, 
TiO2,  MnO,  etc. 

Occurrence,  Uses,  Etc.  While  diorites  in  many  places 
are  found  as  independent  intrusions,  they  are  also  very 
apt  to  be,  on  the  one  hand,  connected  with  granites,  on 
the  other  with  gabbros,  and  usually  these  pass  into  each 
other.  They  also  do  not  form  such  vast  batholiths  or 
stocks  as  the  granites  or  the  gabbros,  and,  especially  as 
independent  masses,  are  more  liable  to  be  found  as  small 
stocks,  large  dikes,  etc.  They  have  a  very  wide  dis- 
tribution and  are  found  in  all  parts  of  the  world  where 
the  older  deeply-seated  igneous  rocks  are  laid  bare  by 
continued  erosion.  In  the  later  formed  mountain  regions 
they  are  also  found  as  stocks  and  dikes.  What  has  been 
said  of  granites  in  this  respect  would  be  largely  true  of 


DESCRIPTION  OF  IGNEOUS  ROCKS  229 

diorite.  Owing  to  its  dark  color  diorite  is  not  so  exten- 
sively used  for  architectural  purposes  as  granite,  though, 
so  far  as  strength,  durability  and  capacity  for  receiving  a 
high  polish  is  concerned,  it  would  furnish  in  many  places 
excellent  material.  It  is  somewhat  heavier  than  granite, 
its  specific  gravity  ranging  from  2.8-3.1;  at  3.0  a  cubic  foot 
of  it  would  weigh  about  187  pounds. 

Relation  to  Other  Rocks.  As  mentioned  under  its 
description  as  a  rock  mineral,  pyroxene  through  meta- 
morphic  processes  changes  into  hornblende.  Generally 
this  is  accompanied  in  the  rock  by  the  production  of 
schistosity  as  described  under  metamorphism  and  horn- 
blende schist.  The  description  of  uralite  should  also  be 
consulted.  It  sometimes  happens  that  this  change  takes 
place  in  gabbro  without  causing  the  rock  to  lose  its 
massive  character  or  becoming  schistose.  In  this  case, 
if  of  sufficiently  coarse  grain  to  permit  the  recognition  of 
the  hornblende  and  feldspar,  it  would  be  classed  as  a 
diorite.  If  it  can  be  proved  that  a  diorite  has  been 
derived  from  gabbro,  it  may  well  be  termed  a  metadiorite 
to  indicate  its  secondary  origin.  Usually,  however,  the 
grain  of  such  rocks  is  quite  fine,  too  much  so  to  permit  the 
individual  hornblende  prisms  to  be  definitely  determined, 
and  the  rock  would  be  classed  under  dolerite.  They  are 
very  apt  to  have  a  green  color  and  for  this  reason  have 
been  called  greenstones.  The  green  color  is  partly  due 
to  hornblende,  partly  to  chlorite.  These  rocks  are 
further  mentioned  under  dolerite. 

GABBRO. 

Composition.  The  gabbros  are  granular  igneous  rocks 
consisting  chiefly  of  pyroxene  and  feldspar  of  any  kind,  in 
which  the  amount  of  pyroxene  equals  or  exceeds  that  of 
the  feldspar.  Usually  more  or  less  iron  ore  in  black 
metallic-looking  grains  can  be  seen,  and  in  some  varieties 
considerable  olivine  may  occur.  This  latter  can  some- 
times be  detected  with  the  lens  as  yellowish  or  green 


230  ROCKS  AND  ROCK  MINERALS 

grains.  Careful  inspection  will  often  show  occasional 
bronzy  flakes  of  biotite.  Of  the  two  chief  minerals  the 
pyroxene  is  usually  dark  greenish  when  examined  with 
the  lens,  often  black  to  the  eye  alone,  and  sometimes  it  is 
of  the  variety  diallage  with  a  pronounced  apparent 
cleavage  in  one  direction,  of  a  gray-green  color  and  often 
almost  micaceous  appearance,  at  times  somewhat  brassy 
or  semi-metallic  in  luster.  A  test  with  the  knife  point  for 
cleavage  shows  at  once  its  non-micaceous  character.  The 
feldspar  in  the  great  majority  of  cases  is  a  soda-lime 
variety,  generally  labradorite,  as  may  often  be  seen  by 
the  twinning  striations  on  a  cleavage  surface.  It  is 
usually  in  formless  masses  or  grains  like  the  other  minerals, 
but  not  unfrequently  it  has  a  tabular  or  book-like  form 
and  the  sections  on  the  rock  face  have  an  elongated  shape. 
In  this  case  the  striations  run  parallel  with  the  elongation. 
Sometimes  the  feldspar  is  fresh  and  glassy;  in  this  case  the 
two  feldspar  cleavages  are  good  and  the  striations  if 
visible  are  distinct;  sometimes  the  feldspar  is  waxy  in 
appearance,  of  a  glimmering  luster  to  dull,  often  with  a 
bluish  tone;  in  this  case  the  cleavage  is  poor  or  even 
apparently  wanting  and  striations  cannot  be  seen.  In 
the  latter  case  the  feldspar  is  more  or  less  affected  by 
alteration  to  other  minerals  as  described  on  page  44. 

A  distinction  is  made  by  petrographers  by  which  gabbros  are 
divided  into  two  groups,  depending  on  the  variety  of  pyroxene 
present.  If  this  is  the  monoclinic,  lime-bearing  augite  or  diallage, 
the  rock  is  called  gabbro,  if  it  is  the  orthorhombic  hypersthene  which 
is  without  lime  the  rock  is  called  norite.  This  distinction  cannot 
be  made  in  megascopic  determinations  unless  some  of  the  pyroxene 
is  extracted  from  the  rock  and  tested  chemically,  hence  the  norites 
are  here  included  under  gabbro.  A  rarer  type  consists  of  plagioclase 
and  olivine  without  pyroxene  and  is  called  Troctolite.  Some  rare 
rocks  with  alkalic  feldspar  are  also  here  included  under  gabbro  which 
are  variously  classified  and  named  by  petrographers. 

General  Properties.  The  color  of  gabbros  is  usually 
dark,  dark  gray  or  greenish  to  black;  very  rarely  reddish. 
In  some  varieties  in  which  diallage  is  the  kind  of  pyroxene 


DESCRIPTION  OF  IGNEOUS  ROCKS 


231 


present  and  the  grain  is  moderately  coarse  the  rock  is 
much  lighter  in  tone  and  of  a  medium  gray  or  greenish- 
gray.  The  same  is  true  in  many  cases  where  the  rock  is 
more  or  less  altered;  compare  with  what  is  said  of  the 
feldspars  above.  The  texture  is  granitoid  or  granular, 
sometimes  with  a  porphyritic  tendency  from  the  elonga- 
tion of  the  feldspars,  but  true  porphyritic  texture  is  very 
rare.  Miarolitic  cavities  are  much  less  frequent  than  in 
granite  and  syenite.  Orbicular  gabbro  has  been  found 
in  California.  A  fluidal  or  banded  structure  which  is 
produced  by  drawn-out  layers  of  varying  composition  and 
which  simulates  a  gneissoid  structure  has  been  described 
from  several  localities,  from  the  Hebrides,  California  and 
Minnesota.  Pegmatites  are  also  occasionally  found  in 
gabbros;  they  consist  of  the  usual  minerals  of  the  rock. 
In  South  Norway  the  pneumatolytic  processes  attending 
the  intrusion  of  gabbros  have  formed  much  scapolite  and 
other  minerals  in  the  gabbro  at  its  border  and  in  dikes  in 
the  contact  zone;  of  these  minerals  apatite  is  the  most 
prominent  and  occurs  sometimes  in  large  masses.  Com- 
plementary dikes,  etc.,  occur  in  gabbro  masses  but  are  not 
perhaps  so  notable  a  feature  as  in  the  foregoing  groups. 
In  this  connection  what  is  said  concerning  peridotites  may 
be  consulted. 

ANALYSES    OF   GABBRO. 


SiO2 

A1203 

Fe203 

FeO 

MgO 

CaO 

Na2O 

KjO 

H20 

XyO 

Total. 

I 

47.9 

18.9 

1.4 

10.5 

7.1 

8.4 

2.7 

0.8 

0.6 

1.7 

100.0 

II 

48.9 

8.8 

1.0 

9.5 

15.2 

14.7 

0.6 

0.1 

0.6 

0.7 

100.1 

III 

49.9 

18.5 

2.1 

8.4 

5.8 

9.7 

2.6 

0.7 

1.0 

1.4 

100.1 

TV 

52.8 

17.8 

1.2 

4.8 

4.8 

12.9 

3.0 

0.5 

1.2 

0.5 

99.5 

V 

40.2 

9.5 

9.7 

12.2 

8.0 

13.1 

0.8 

0.2 

0.5 

5.5 

99.7 

I,  Adirondacks,  New  York  State;  II,  Orange  Grove,  Maryland; 
III,  Pigeon  Point,  Minnesota;  IV,  Band  rich  in  feldspar,  poor  in 
pyroxene,  Isle  of  Skye,  Hebrides.  V,  Band  rich  in  pyroxene,  poor 
in  feldspar,  Isle  of  Skye.  XyO = small  quantities  of  various  oxides, 
chiefly  TiO2. 


232  ROCKS  AND  ROCK  MINERALS 

Chemical  Composition.  The  gabbros,  as  a  rule,  contain 
larger  amounts  of  lime,  iron  and  magnesia,  and  less  of 
silica  and  alkalies  than  any  of  the  previously  described 
rocks  as  may  be  seen  from  the  table  annexed. 

Analyses  IV  and  V  show  how  the  chemical  composition 
of  the  banded  gabbros  varies  in  the  different  streaks  with 
corresponding  variation  in  mineral  contents. 

Occurrence.  Gabbros  are  widely  distributed  and  com- 
mon rocks.  They  are  found  as  large  stocks  and  bath- 
yliths  and  in  dikes  in  the  older  rock  complexes,  similarly  to 
granite.  They  are  also  found  as  stocks  and  necks  of  old 
volcanoes  cutting  the  stratified  beds  of  the  younger  moun- 
tain regions.  In  these  they  may  also  be  found  as  thick 
intrusive  sheets.  Gabbros  have  been  held  to  occur  also 
as  forming  the  central  portion  of  thick  extrusive  sheets, 
as  in  the  Hebrides,  in  Sweden  and  in  the  Lake  Superior 
region.  If  this  is  the  case  it  is  due  to  the  low  freezing 
point  of  the  magma,  its  liquidity  and  ready  crystallization. 

In  the  United  States,  gabbros  are  found  in  many  places 
in  New  England,  as  in  the  White  Mountains.  They  are 
found  in  the  Adirondacks  and  at  Cortlandt  on  the  Hudson 
River  in  New  York  State,  in  Maryland,  etc. 

They  occur  in  the  Lake  Superior  region  and  elsewhere 
in  Minnesota  and  in  various  places  in  the  Rocky  Moun- 
tains and  in  California.  They  are  extensively  distributed 
in  Europe,  in  southern  England,  in  northern  Scotland, 
especially  on  the  islands  of  the  Hebrides,  in  Norway  and 
Sweden  and  in  Germany.  They  are  in  fact  almost  as 
widely  known  as  granites  though  they  do  not  form,  as  a 
rule,  such  large  masses. 

Alteration  of  Gabbro.  It  is  common  to  find  that  where 
gabbro  massives  occur  in  the  older  rock  complexes  and  in 
folded  mountain  ranges  that  they  are  surrounded  by  a 
mantle  of  hornblende  schist  into  which  the  gabbro  grad- 
ually passes,  by  transitional  phases.  The  origin  of  this 
is  the  pressure,  shearing  and  other  metamorphic  agencies 
brought  about  by  the  orogenic  processes,  as  mentioned 


DESCRIPTION  OF  IGNEOUS  ROCKS  233 

under  metamorphism,  hornblende,  diorite,  etc.,  which 
have  acted  upon  the  pyroxene  of  the  gabbro  converting 
it  into  hornblende  and  producing  the  schistose  structure. 
It  may  happen  through  pressure  and  shearing  that  a 
schistose  or,  perhaps  better,  a  gneissoid  structure  may  be 
induced  in  the  gabbro  without  change  of  the  pyroxene 
to  hornblende  and  we  would  have  in  this  case  a  gabbro- 
gneiss  or  gabbro-schist  produced,  but  generally  the  change 
to  hornblende  occurs.  If  olivine  is  present  it  also  forms 
amphibole.  Very  often  garnet  appears  as  a  new  mineral 
resulting  from  the  process.  While  the  change  to  horn- 
blende is  usually  accompanied  by  the  assumption  of  a 
more  or  less  pronounced  gneissoid  or  schistose  structure, 
this  is  not  always  the  case;  the  rock  sometimes  retains  a 
massive  granular  character  and,  if  its  constituent  feldspar 
and  hornblende  can  be  recognized,  it  would  be  classed  as  a 
diorite,  as  mentioned  under  that  rock.  In  another  mode 
of  alteration  of  gabbros  the  feldspar  is  changed  into  a 
substance  called  saussurite,  which  was  formerly  thought 
to  be  a  distinct  mineral,  but  which  the  microscope  has 
shown  to  be  a  mixture  of  albite,  zoisite  and  other  minerals. 
The  feldspar,  or  rather  that  which  replaces  it,  has  no 
cleavage  and  is  waxy  looking.  The  pyroxene  is  changed 
to  hornblende,  which  tends  to  have  a  bright  to  grass- 
green  color  and  is  the  variety  called  smaragdite.  Other 
minerals  are  also  formed,  but  megascopically  the  waxy- 
looking  saussurite  and  green  hornblende  predominate. 
This  may  take  place  without  formation  of  schistose 
structure  and  it  seems  probable  that  in  this  case  the 
alteration  is  due  more  to  the  chemical  and  less  to  the 
dynamic  agencies  of  metamorphism.  Such  rocks  have 
been  called  saussurite-gabbro. 

In  the  process  of  weathering  through  the  agencies  of 
the  atmosphere,  gabbros  give  rise  to  clay  soils  deeply 
colored  by  the  oxides  of  iron  and  mingled  with  fragments 
of  still  undecomposed  minerals. 

Iron  and  Other  Ore  Deposits.    There  are  frequently 


234  ROCKS  AND  ROCK  MINERALS 

found  in  large  gabbro  intrusions  masses  of  iron  ore, 
sometimes  consisting  of  magnetite,  but  generally  of 
ilmenite  or  mixtures  of  the  two.  Usually  these  are  more 
or  less  mingled  with  the  minerals  of  the  gabbro  itself, 
especially  pyroxene  and  olivine.  The  character  of  the 
occurrences,  their  lack  of  definite  form  and  the  manner  in 
which  they  gradually  shade  into  the  normal  gabbro,  show 
that  they  are  only  a  phase  of  the  rock  in  which  the  iron 
ore,  usually  scattered  through  it  in  small  grains,  is  here 
locally  concentrated  in  great  abundance.  Such  ore  de- 
posits are  sometimes  found  at  the  border  of  the  intrusion, 
though  often  scattered  in  masses  through  it  or  at  the 
center.  They  are  known  in  many  places,  in  the  Adiron- 
dacks,  in  northern  Minnesota,  in  Canada,  Norway,  Sweden 
and  elsewhere.  If  titaniferous  iron  ores  could  be  success- 
fully smelted,  such  deposits  would  undoubtedly  be  in 
many  cases  of  great  value. 

In  other  cases  sulphide  ores  are  developed  in  gaboro 
rocks  in  a  similar  manner.  This  is  especially  true  of  the 
sulphide  of  iron  called  pyrrhotite,  which  is  often  nickel- 
bearing  and  hence  of  great  value  as  a  source  of  this  useful 
metal.  In  some  places  these  deposits  are  accompanied 
by  valuable  amounts  of  copper  in  the  form  of  chalcopyrite, 
copper-iron-pyrites,  and  it  has  been  remarked  that  as  the 
percentage  of  copper  rises  that  of  nickel  falls.  Such 
deposits  in  gabbros,  or  in  rocks  derived  from  them,  are 
known  and  have  been  worked  in  Norway  and  Sweden,  in 
Lancaster  County,  Penn.,  and  at  Sudbury,  Ontario. 

The  origin  of  this  kind  of  ore  deposit  in  an  igneous  rock 
has  been  described  on  page  170. 

Use  of  Gabbro.  The  gabbros  are  well  suited  for  con- 
structional work  and  architecture,  but  as  a  rule  have  not 
been  extensively  used,  probably  very  largely  on  account 
of  their  dark  color.  In  Sweden  they  have  received  con- 
siderable attention  for  monumental  and  other  uses.  In 
the  United  States  they  have  been  used  for  building  in  the 
Lake  Superior  region,  as  at  Duluth,  and  quarries  of  them  at 


DESCRIPTION  OF  IGNEOUS  ROCKS  235 

Keeseville  in  the  Adirondacks  and  in  Vergennes,  Vermont, 
have  been  worked.  They  take  a  high  polish,  are  suf- 
ficiently durable  and  much  easier  to  work  than  granite. 

DOLERITE. 

Definition  and  Minerals.  The  dolerites,  as  already 
explained  in  the  section  on  classification,  comprise  those 
forms  of  diorite  and  gabbro  in  which,  generally  on  ac- 
count of  increasing  fineness  of  grain,  the  hornblende 
and  pyroxene  cannot  be  safely  determined  or  distin- 
guished from  one  another,  although  the  eye  or  lens 
clearly  sees  that  the  rock  is  composed  of  feldspar  min- 
gled with  an  equal  or  greater  amount  of  a  ferromagnesian 
mineral. 

This  term  as  here  used  comprises  not  only  the  finer-grained 
diorites  and  gabbros  but  much  also  of  what  is  termed  "  diabase  " 
by  the  petrographers,  as  well  as  occasional  rare  rocks  which  need  no 
mention  here. 

The  feldspar,  which  is  seen  in  larger  or  smaller  grains 
and  sometimes  in  more  or  less  extended  lath-shaped 
sections,  is  known  from  microscopic  studies  to  be  chiefly 
a  soda-lime  variety,  though  alkalic  ones  are  also  present  to 
some  extent  and  in  some  cases  may  replace  the  former; 
these  distinctions  cannot  be  made  megascopically,  and  the 
plagioclase  twinning  can  very  rarely  be  seen  on  a  cleavage 
face  with  the  lens.  The  ferromagnesian  minerals  are  in 
dark  grains,  perhaps  short  columnar;  their  cleavage  sur- 
faces can  usually  be  seen  but  it  cannot  be  said  whether 
they  are  hornblende  or  pyroxene  or  a  mixture  of  both. 
Sometimes  olivine  is  also  present  and  if  its  yellow-green 
grains  can  be  detected  it  is  very  probable  that  pyroxene 
is  the  chief  ferromagnesian  mineral  and  not  hornblende. 
In  addition  the  lens  will  often  show  bronzy-looking 
flakes  of  biotite  and  metallic  steel-like  specks  of  iron 
oxide  (magnetite  or  ilmenite)  or  sometimes  brass-like 
crystals  or  grains  of  pyrite. 


236  ROCKS  AND  ROCK  MINERALS 

Color.  Owing  to  the  equal  or  predominant  amount  of 
ferromagnesian  minerals,  the  color  of  these  rocks  is  dark, 
medium  or  dark  gray  or  greenish  to  black.  As  in  most 
rocks  the  tone  of  color  is  best  observed  in  viewing  the 
rock  at  a  little  distance,  so  that  the  individual  grains 
become  indistinguishable  and  only  their  mass  effect  is 
seen. 

General  Properties.  The  texture  of  these  rocks  is  granu- 
lar to  fine  granular,  they  are  sometimes  porphyritic  but 
these  cases  are  described  in  the  following  section  on  the 
porphyries.  Their  chemical  composition,  in  the  great 
majority  of  cases,  is  similar  to  that  of  the  diorites  and 
gabbros  already  given  and  need  not  be  repeated.  They 
are  heavy,  the  specific  gravity  being  from  3.0-3.3.  Their 
jointing  is  usually  small  cuboidal,  wedge  shaped  or  platy, 
often  columnar  and  sometimes  on  a  very  large  scale, 
though  generally  this  structure  is  not  so  perfect  as  in  the 
finer-grained  basalts.  It  is  most  apt  to  occur  in  dikes, 
very  thick  intrusive  sheets  and  in  massive  extrusive  flows. 

Occurrence.  The  dolerites  do  not  occur  in  large  stocks 
and  bathyliths  like  the  diorites  and  gabbros,  though  not 
infrequently  these  latter  rocks  pass  into  an  endomorphic 
phase  of  dolerite  at  the  margin  of  the  intrusion.  As  in- 
trusives  they  belong  in  the  minor  class,  being  found  in 
dikes,  small  laccoliths  and  intrusive  sheets,  the  latter  often 
of  great  thickness,  and  in  thick  massive  lava  flows  whose 
cooling  has  been  slow. 

In  the  eastern  United  States  the  most  conspicuous 
examples  are  found  in  the  intrusions  and  flows  of  "  trap  " 
of  the  Triassic  formations  stretching  from  Nova  Scotia  to 
Georgia.  Through  faulting  and  erosion  they  now  give 
rise  to  definite  topographic  features,  such  as  the  ridges  in 
Connecticut  and  the  Palisades  opposite  New  York  City. 
Similar  masses  of  these  rocks  are  found  in  the  Lake  Superior 
region  and  in  the  great  lava  flows  of  the  western  United 
States.  In  all  these  occurrences  they  are  associated  with, 
and  pass  into,  the  denser  forms  of  basalt.  These  larger 


DESCRIPTION  OF  IGNEOUS  ROCKS  237 

occurrences  of  dolerite  mostly  contain  pyroxene  as  the 
dominant  ferromagnesian  mineral  and  are  largely  the 
rock  called  "  diabase  "  by  petrographers,  while  cases  where 
hornblende  is  dominant  are  mostly  confined  to  dikes  and 
smaller  intrusions,  especially  in  the  older  rocks. 

Dolerites  are  also  very  common  rocks  in  Great  Britain 
in  various  localities,  in  dikes  and  intrusive  sheets,  and 
especially  in  the  north  of  Scotland  and  Ireland  where  they 
are  often  extrusive  and  associated  with  denser  basalts. 
They  are  in  fact  very  common  rocks  in  all  parts  of  the 
world. 

Relation  to  Other  Rocks  —  Alteration.  From  what  has 
been  said  it  is  easy  to  see  that  the  dolerites  are  a  class 
of  rocks  based  largely  on  convenience.  On  the  one  hand 
they  form  a  transition  group,  based  on  texture,  between  the 
diorites  and  gabbros  and  the  dense  basalts,  and  on  the  other 
they  cannot  depend  wholly  on  texture,  because  relatively 
coarse-grained  rocks  may  occur  in  which  one  cannot  dis- 
tinguish between  hornblende  and  pyroxene  and  which 
must  therefore  be  placed  in  this  class. 

The  case  might  occur  in  which,  instead  of  hornblende  or  pyroxene, 
biotite  was  the  dominant  mineral  associated  with  the  feldspar. 
Such  rocks  are  not  very  common  but  sometimes  occur,  especially  in 
dikes  and  sheets  and  with  quite  fine  grain.  They  form  the  rocks 
called  mica  trap  or  minette,  mentioned  later  under  basalt. 

The  pyroxenic  members  of  this  group,  by  regional 
metamorphism,  become  converted  into  hornblende  rocks, 
generally  into  hornblende  schists,  and  both  varieties  by 
alteration  may  produce  chlorite  and  pass  into  the  so-called 
"  greenstones."  These  alterations  are  quite  similar  to 
what  has  been  described  under  gabbro.  By  weathering 
they  become  brownish  and  discolored  and  ultimately 
yield  brown  ferrugineous  soils. 

Uses.  The  rocks  of  this  group  are  too  dark  and  somber 
for  general  use  in  fine  architectural  or  interior  work, 
except  for  monumental  purposes.  The  "  trap  "  of  the 


238      »  ROCKS  AND  ROCK  MINERALS 

eastern  states  has  been  considerably  employed  in  rough 
masonry,  and  where  good  natural  joint  faces  can  be  used 
for  wall  surfaces,  the  brown  weathering  color  gives  pleas- 
ing effects.  The  toughness  of  the  material,  which  the 
traps  afford,  has  however  caused  it  to  be  considerably 
used  for  block  paving  and  the  crushed  stone  for  road 
making. 

PERIDOTITE  —  PYROXENITE. 

Composition.  Under  this  group  are  comprised  all  of 
those  granular  igneous  rocks  composed  of  ferromagnesian 
minerals  alone,  or  in  which  the  amount  of  detectible  feld- 
spar is  so  small  as  to  be  entirely  negligible  as  a  component, 
and  in  which  the  mineral  grains  are  sufficiently  large  to  be 
determined.  The  chief  minerals  which  form  these  rocks 
are  olivine,  pyroxene  of  both  the  augite  and  hypersthene 
varieties,  and  hornblende.  These  may  occur  alone  or  in 
various  mixtures,  and  according  to  these  the  group  has 
been  sub-divided  into  types,  some  of  the  more  prominent 
of  which  are  as  follows: 

Pyroxenes  and  Olivine Peridotite. 

Hornblende  and  Olivine Cortlandtite. 

Olivine  alone Dunite. 

Pyroxenes  alone Pyroxenite. 

Hornblende  alone Hornblendite. 

The  first  three,  which  contain  olivine,  are  comprised 
under  the  general  name  of  peridotites,  from  peridot,  the 
French  word  for  olivine.  But  all  the  different  types, 
while  they  sometimes  occur  independently,  also  occur 
together,  with  transition  forms  grading  into  one  another, 
and  it  is  difficult,  and  sometimes  impossible,  to  distinguish 
them  megascopically  and  therefore  they  are  best  treated 
together  as  one  general  group  and  not  as  separate  rocks. 

Beside  the  minerals  mentioned,  a  brown  biotite  some- 
times occurs  in  these  rocks,  giving  rise  to  the  variety  called 
mica  peridotite.  Additional  accessory  minerals,  some  of 
which  are  common  and  some  confined  to  certain  occur- 


DESCRIPTION   OF  IGNEOUS  ROCKS 


239 


rences,  are  titanic  iron  ore,  spinels,  of  which  chromite  is  of 
importance,  and  garnet. 

Texture.  The  texture  is  granitoid  or  granular;  its 
appearance  depends  somewhat  on  the  minerals  present 
and  their  arrangement.  When  pyroxene  or  hornblende 
is  the  dominant  mineral  the  grain  is  often  very  coarse  and 
may  exhibit  large  cleavage  surfaces.  Dunite  is  not  apt 
to  be  coarse  grained;  it  commonly  has  a  sugar-granular 
texture  like  many  aplites,  sandstones,  marbles,  etc. 
Porphyritic  texture  is  rare  or  wanting.  A  common 
texture  is  one  in  which  the  cleavage  surfaces  of  the 
pyroxenes  or  hornblendes  are  seen  to  be  spotted  with 
grains  of  olivine  included  in  the  larger  crystal.  Such  a 
spotting  of  the  shining  cleavage  surfaces  of  one  mineral 
by  smaller  included  crystals  of  another,  which  have  no 
crystal  orientation,  either  with  respect  to  one  another  or  to 
their  host,  is  called  luster  mottling  and  is  known  as  the 
poikilitic  texture.  It  is  sometimes  well  exhibited  in  these 
rocks.  The  included  crystals  are  of  course  older  than 
their  host. 

Chemical  Composition.  This  varies  according  to  the 
minerals  in  the  rocks  but  general  characters  are  the  very 
low  silica,  the  small  amount  or  virtual  absence  of  alkalies 
and  alumina,  and  the  large  quantities  of  iron  and  magnesia. 

ANALYSES    OF    PERIDOTITES.    ETC. 


SiO, 

AljO, 

Fe203 

FeO 

MgO 

CaO 

NajO 

KjO 

H,0 

XyO 

Total. 

I 
II 
III 

TV 

40.1 
43.9 
53.2 
46  4 

7.8 
1.6 
1.9 
10  8 

7.3 
8.9 
1.4 
5.9 

8.6 
2.6 
7.9 
5.6 

23.7 
27.3 
20.8 
22.2 

6.5 
6.3 
13.1 
3  7 

1.2 
0.5 
0.1 
0.3 

0.5 

b.i 

1  fl 

4.0 

8.7 
1.0 
3.8 

0.6 
0.3 
0.5 

100.3 
100.1 
100.0 
100.1 

V 

38.4 

0.3 

3.4 

6.7 

45.2 

0.4 

0.1 

4.3 

1.4 

100.2 

I,  Peridotite,  Devonshire,  England;  II,  Peridotite,  Baltimore 
County,  Maryland;  III,  Pyroxenite,  Oakwood,  Maryland;  IV,  Horn- 
blendite,  Valbonne,  Pyrenees;  V,  Dunite,  Tulameen  River,  British 
Columbia.  XyO  =  small  quantities  of  other  oxides. 


240  ROCKS  AND  ROCK  MINERALS 

Color.  The  color  of  these  rocks  ordinarily  varies  from 
dull  green  to  black.  The  dunites,  which  are  practically 
composed  of  the  one  mineral  olivine,  are  at  times  much 
lighter.  They  may  show  various  shades  of  light  green, 
medium  yellow  and  light  brown,  passing  into  one  another, 
and  from  these  through  dull  yellowish  green  into  dark 
green.  They  may  thus  be  exceptions  to  the  general  rule 
that  ferromagnesian  rocks  are  dark  colored. 

Occurrence.  —  Relation  to  Gabbro.  The  peridotites 
and  allied  rocks  sometimes  occur  independently  as  dikes, 
sheets,  laccoliths  or  small  intrusive  stocks.  In  this  way, 
as  small  isolated  occurrences  they  have  been  found  cutting 
the  Paleozoic  rocks,  usually  in  a  more  or  less  altered  con- 
dition, at  Syracuse  and  other  localities  in  New  York  State, 
in  Kentucky,  in  Arkansas  and  elsewhere.  But  generally 
speaking  they  are  most  liable  to  occur  in  connection  with 
greater  intrusions  of  gabbros.  Sometimes  they  form  phases 
of  the  gabbro  mass,  with  transitions  between  the  two;  some- 
times they  cut  the  gabbros  in  dikes  or  are  found  in  small 
intrusions  in  their  neighborhood.  This  dependence  upon 
the  gabbros  has  led  to  their  being  held  in  such  cases  as 
products  of  differentiation  of  the  gabbro  magma  in  which 
they  represent  the  lamprophyres  of  other  rock  groups. 
In  this  way  a  great  number  of  occurrences  are  known 
in  all  parts  of  the  world  where  gabbros  are  common 
rocks. 

Dunites  occur  in  masses  intrusive  in  the  gneisses  of 
western  North  and  South  Carolina  and  Georgia.  Asso- 
ciated with  them  are  smaller  amounts  of  other  peridotites 
and  pyroxenite.  These  occurrences  are  of  importance  on 
account  of  the  deposits  of  corundum  of  commercial  value 
associated  with  them.  The  mineral  is  thought  to  have 
formed  in  them  in  the  same  manner  as  described  under 
syenite.  Dunite  also  occurs  in  considerable  masses  in  New 
Zealand,  especially  in  the  Dun  Mountains,  from  which  came 
the  name.  Pyroxenite  and  hornblendite  are  compara- 
tively rare  and  of  relatively  small  geologic  importance. 


DESCRIPTION  OF  IGNEOUS  ROCKS  241 

Alteration.  —  Serpentine.  The  peridotites  are  extremely 
liable  to  alteration,  so  much  so  that  unchanged  occurrences 
are  not  at  all  usual.  The  most  common  form  of  alteration 
is  that  in  which  the  olivine  and  other  magnesian  silicates 
are  changed  to  serpentine. 

This  is  illustrated  by  the  following  reactions: 

Olivine       Enstatite        Water  Serpentine 

Mg2SiO4  +  MgSiO3  +  2  H2O  =  H4Mg3Si2O9 

2  Mg2SiO4  +  CO2        +2  H2O  =  H4Mg3Si.A  +  MgCO3 

3  MgSiO3   +  2  H2O    =  H4Mg3Si2O9  +  SiO2. 

Other  magnesian  minerals  such  as  talc  are  also  formed 
by  the  alteration  of  these  rocks,  but  that  to  serpentine 
is  the  most  important.  All  stages  of  transition  to  pure 
serpentine  occur,  and  studies  which  have  been  made  in 
recent  years  show  that  a  large  part,  perhaps  the  greater 
part,  of  the  occurrences  of  this  mineral  are  to  be  assigned 
to  the  alteration  of  rocks  of  this  group. 

The  peridotites  ultimately  weather  down  into  brown 
ferrugineous  soils  which,  on  account  of  their  lack  of  potash, 
do  not  favor  vegetable  growth  and  are  therefore  barren. 

As  a  Source  of  Valuable  Minerals.  The  magmas  which 
form  the  peridotites  usually  carry  small  amounts  of 
chromic  oxide  which  often  crystallizes  with  iron  oxide  to 
form  the  mineral  chromite,  FeCr2O4,  one  of  the  spinel 
group.  It  is  often  seen  in  dunite  and  usually  forms  small, 
black,  pitchy-looking  grains.  Sometimes  this  mineral  is 
concentrated  in  sufficient  amount  so  that  it  becomes  a 
useful  ore,  supplying  the  chromium  used  in  the  arts. 

The  olivine  of  these  rocks  has  been  found  by  analysis  to 
contain  a  minute  amount  of  nickel  oxide;  when  they 
change  to  serpentine  it  sometimes  happens  that  this 
nickel  is  concentrated  in  the  form  of  nickel  silicate,  some- 
times in  amounts  sufficient  to  form  deposits  of  value  as  a 
source  of  this  metal,  as  in  Douglas  County,  Oregon,  and 
in  the  Island  of  New  Caledonia. 

The  peridotites,  and  to  some  extent  their  allies  the 


242  ROCKS  AND  ROCK  MINERALS 

gabbros,  are  also  the  source  of  platinum,  which  occurs  in 
them  as  the  native  metal  or  as  sperrylite  PtAs2j  by  the 
decay  of  the  rock  it  is  washed  down  and,  like  gold,  concen- 
trated in  alluvial  deposits.  The  precious  garnet,  pyrope, 
used  as  a  gem,  also  comes  from  a  decayed  and  serpentinized 
peridotite  from  Bohemia,  South  Africa,  etc.  Lastly,  the 
diamonds  of  South  Africa  have  their  source  in  decayed 
and  greatly  altered  peridotite  rocks.  This  altered  rock, 
which  was  originally  a  mica  peridotite,  is  known  as  kim- 
berlite,  by  the  miners  as  "  blue  ground."  Some  have  held 
that  the  carbon  forming  the  diamonds  was  derived  from 
the  shales  through  which  the  magma  passed,  others  hold 
that  it  was  original  in  the  magma  and  that  the  diamond 
is  a  true  crystalline  constituent  of  the  igneous  rock  like 
any  other  of  its  accessory  minerals. 

PORPHYRIES. 

Definition.  As  explained  in  the  former  section  treating 
of  the  classification  of  porphyries,  these  rocks  may  be 
divided  into  two  main  groups;  one  in  which,  on  account  of 
its  coarse  texture,  not  only  the  phenocrysts  but  the  grains 
of  the  groundmass  can  be  determined  or  the  determinable 
phenocrysts  form  so  large  a  proportion  of  the  rock  that  a 
good  idea  of  its  mineral  composition  can  be  obtained  and 
the  small  amount  of  dense  groundmass  may  be  neglected, 
and  a  second  group  in  which  the  amount  of  dense  ground- 
mass  is  large  and  the  phenocrysts  are  not  abundant 
enough  to  determine  safely  the  mineral  character  of  the 
rock.  It  is  the  first  of  these  two  groups  which  is  described 
in  this  section,  the  one  which  we  may  call  the  group  of 
determinable  porphyries;  the  second  group  will  be  con- 
sidered later  in  connection  with  the  dense  igneous  rocks  — 
the  felsites  and  basalts  —  of  which  they  form  a  por- 
phyritic  variety. 

In  this  first  group,  porphyries  are  mainly  confined  to  the 
feldspathic  division  of  the  igneous  rocks,  apparently  for 
the  reason  that  the  magmas  which  furnish  the  ferro- 


PLATE  19. 


A.   GRANITE-PORPHYRY,    MONTANA. 


B.   SYENITE-PORPHYRY, 
MONTANA. 


C.   SYENITE-PORPHYRY, 
NORWAY. 


DESCRIPTION  OF  IGNEOUS  ROCKS  243 

magnesian  rocks  have  relatively  so  low  a  freezing  point 
and  crystallize  so  readily  that  they  are  not  apt  to  form 
porphyries  under  conditions  where  the  feldspathic  rocks 
often  do  so  readily.  Thus  granite  porphyry  is  very  com- 
mon, while  gabbro  and  peridotite  porphyries  are  so  rare 
as  to  be  of  no  practical  importance.  In  the  group  of 
dense  igneous  rocks  porphyries  of  both  divisions  are  com- 
mon. The  rocks  to  be  treated  then  are  granite  porphyry, 
syenite  porphyry  and  diorite  or  dolerite  porphyry.  There 
are  so  many  points  in  which  they  are  similar  that  they 
are  best  treated  as  a  group. 

Granite  Porphyry.  This  consists  of  distinct  pheno- 
crysts  of  quartz  and  of  feldspar  in  a  granular  groundmass 
of  the  same  minerals  whose  grains  can  be  determined  as 
such,  or  one  in  which  the  abundance  of  the  phenocrysts  of 
quartz  and  feldspar  give  a  distinct  granite-like  character 
to  the  rock  and  make  the  dense  groundmass  of  less  impor- 
tance. Sometimes  the  rock  consists  of  these  minerals 
alone,  or  very  nearly  so,  and  sometimes  biotite  and  horn- 
blende are  present,  perhaps  in  considerable  amount.  The 
biotite  and  hornblende  may  be  present  separately  or 
together,  though  hornblende  alone  is  rare.  They  may 
occur  as  distinct  phenocrysts,  usually  smaller  than  the 
quartz  and  especially  the  feldspar,  and  also  in  the  ground- 
mass,  in  which  case  the  tiny  specks  of  biotite  are  most 
easily  detected. 

When  the  groundmass  is  so  coarse  as  to  be  equivalent  to  an 
ordinary  granite  it  is  customary  to  speak  of  the  rock  as  porphyritic 
granite,  as  explained  under  granite. 

Syenite  Porphyry.  This  rock  consists  of  distinct  pheno- 
crysts of  feldspar  in  a  groundmass,  which,  if  determinable, 
must  be  made  up  mainly  of  grains  of  feldspar  and  with 
very  little  or  no  quartz.  If  the  groundmass  is  not  deter- 
minable the  amount  of  phenocrysts  must  be  large  enough 
to  give  the  rock  a  distinctly  syenitic  character.  The 
ferromagnesian  minerals,  biotite,  hornblende  and  pyroxene, 


244        ROCKS  AND  ROCK  MINERALS 

while  they  may  be  absent  or  practically  so,  are  usually 
present,  either  as  phenocrysts,  or  in  the  groundmass,  or 
both.  They  may  occur  in  considerable  amount,  but  must 
not  equal  or  exceed  the  total  amount  of  feldspar,  or  the 
rock  becomes  a  diorite  porphyry.  They  occur  separately 
and  together,  but  the  combination  of  all  three  or  of 
biotite  and  pyroxene  is  not  so  common  as  biotite  and 
hornblende. 

The  rock  defined  above  is  that  which  corresponds  to  the  common 
one  of  the  three  varieties  of  syenite  described  on  page  219  and 
following,  and  represents  it  in  porphyritic  development.  Anortho- 
site  porphyry  is  unknown.  Nephelite  syenite  porphyry  is  known 
but  is  a  very  rare  rock. 

In  more'  exact  classification  based  on  microscopic  research  a 
distinction  is  made  as  to  whether  the  feldspars  are  chiefly  alkalic 
or  mainly  soda-lime  feldspars,  both  phenocrysts  and  groundmass 
being  considered  together.  In  the  latter  case  petrographers  term 
the  rock  diorite  porphyry  and  only  apply  the  term  of  syenite  por- 
phyry where  they  are  mainly  alkalic.  So  far  as  the  groundmass  is 
concerned  this  distinction  cannot  be  made  by  megascopic  exam- 
ination and  but  rarely,  as  described  later,  with  the  phenocrysts. 
Hence,  just  as  in  the  case  of  syenite,  both  kinds  are  classed  here 
together. 

Diorite  and  Dolerite  Porphyry.  Diorite  porphyry  would 
be  composed  of  phenocrysts  of  hornblende  and  feldspar, 
either  separately  or  together,  in  a  determinable  ground- 
mass  of  the  same  minerals,  or  if  the  groundmass  is  not 
determinable  the  diorite  character  must  be  clearly  shown 
by  the  great  abundance  of  the  hornblende  and  feldspar 
phenocrysts.  Also  the  total  amount  of  hornblende  must 
equal  or  exceed  that  of  the  feldspar.  Some  biotite  may 
also  be  present,  as  well  as  iron  ore  grains. 

Such  rocks  occur  and  it  may  be  possible  at  times  to 
determine  them  megascopically,  but  in  the  great  majority 
of  instances  it  will  be  found  that,  while  the  hornblende 
which  is  present  in  phenocrysts  may  be  recognized,  that 
which  is  present  in  the  groundmass  cannot.  It  can  often 
be  seen  in  these  cases  that  the  groundmass  is  composed  of 


DESCRIPTION  OF  IGNEOUS  ROCKS  245 

feldspar  and  a  ferromagnesian  mineral,  either  hornblende 
or  pyroxene,  but  megascopically  it  is  impossible  to  say 
which.  In  fact  such  groundmasses  correspond  to  the 
definition  and  description  of  dolerite  previously  given  and 
such  rocks  therefore  are  most  conveniently  called  dolerite 
porphyry.  The  phenocrysts  are  either  feldspar,  horn- 
blende, or  pyroxene,  or  mixtures  of  them.  The  feldspar  in 
these  rocks  is  generally  a  variety  of  the  soda-lime  group, 
usually  labradorite.  By  increase  in  the  amount  and 
density  of  the  groundmass  they  pass  insensibly  into  the 
basalt  porphyries,  or  melaphyres,  described  later. 

Phenocrysts  of  Porphyries.  As  the  phenocrysts  of 
porphyries  have  crystallized  freely  in  the  fluid  magmas 
they  generally  show  distinct  crystal  shapes,  such  as  are 
described  in  the  foregoing  part  devoted  to  the  rock 
minerals.  A  few  words  in  regard  to  their  crystal  habits 
may  be  added  here.  Quartz,  as  a  phenocryst,  tends  to 
take  the  form  shown  in  Fig.  43,  but  is  usually  spherical; 
the  crystals  may  be  a  half  inch  in  diameter  but  are  usually 
much  smaller,  the  size  of  coarse  shot  or  peas;  it  is  usually 
smoky  in  color.  The  feldspars  tend  to  assume  the  forms 
shown  by  Figs.  5-7;  they  are  often  twins,  Fig.  8;  they 
are  white,  pink  to  red,  or  yellowish  and  gray;  if  feldspars 
of  two  colors  are  present  and  one  of  them  is  a  reddish 
tone  it  is  probably  orthoclase,  the  other  albite  or  a  soda- 
lime  feldspar.  They  not  infrequently  form  very  large 
phenocrysts,  an  inch  or  even  more  in  length;  the  model- 
like  feldspars  seen  in  mineral  cabinets  often  are  the  pheno- 
crysts obtained  from  porphyries.  Hornblende  occurs  in 
dark  greenish  or  black  prisms,  usually  elongated,  and  some- 
times quite  slender  and  with  glittering  cleavage  surfaces 
if  fresh;  the  terminal  faces  are  poor  or  wanting;  some- 
times it  is  weathered  out  and  only  a  rusty  mass  left  in 
its  place.  Pyroxene  is  also  dark  green  to  black,  in  short, 
stout  prisms,  and  commonly  its  cleavage  and  crystal  faces 
lack  the  luster  of  hornblende.  The  method  of  distil 
guishing  them  has  been  already  explained.  Rusty  spots 


246  ROCKS  AND   ROCK  MINERALS 

also  show  the  former  presence  of  pyroxene,  but  less  com- 
monly than  hornblende.  In  size  both  are  apt  to  be  small, 
compared  with  feldspar.  Biotite  as  a  phenocryst,  is  in  six- 
sided  tablets  with  fine  basal  cleavage,  black  to  bronze- 
brown  in  color.  In  these  rocks  its  crystals  are  apt  to  be 
small. 

General  Properties.  The  chemical  composition  of  these 
porphyries  is  similar  to  that  of  the  corresponding  kinds  of 
granular  rocks  previously  given  and  need  not  be  repeated. 
Their  specific  gravity  and  modes  of  alteration  and  conver- 
sion into  soil  are  the  same.  The  jointing  depends  largely 
on  the  mode  of  occurrence ;  it  is  apt  to  be  platy  or  small 
cuboidal,  or  to  form  small  parallelopipedons  with  acute 
angles,  in  the  feldspathic  porphyries  of  dikes  and  sheets, 
and  larger  blocks  in  the  greater  intrusions;  the  doleritic 
porphyries  tend  to  columnar  jointing. 

Occurrence.  The  porphyries  of  this  class  are  commonly 
found  in  the  minor  intrusions;  in  dikes,  intrusive  sheets 
and  laccoliths,  sometimes  in  volcanic  necks.  They  are 
also  not  uncommon  as  marginal  phases  of  intrusive  stocks 
and  bathyliths  of  granite,  syenite,  etc.;  they  here  represent 
an  endomorphic  contact  modification  and  in  traversing 
areas  of  such  rocks,  if  it  is  observed  that  they  are  becom- 
ing porphyries  with  finer  grain,  approach  to  the  contact 
should  be  suspected.  They  may  also  occur  in  extrusive 
lava  flows,  especially  if  these  are  very  thick  and  massive, 
but  in  this  mode  of  occurrence  they  are  generally  replaced 
by  the  denser  felsite  and  basalt  porphyries  described 
beyond. 

These  rocks  are  far  too  common  to  give  any  list  of 
localities;  they  are  everywhere  found  where  erosion  has 
exposed  the  older  crystalline  rocks  and  where  igneous 
activity,  has  displayed  itself.  Where  larger  stocks  and 
intrusions  have  occurred  they  are  especially  apt  to  be 
present,  sometimes  cutting  them  as  dikes,  sometimes 
extending  from  them  in  apophyses,  and  sometimes  in 
dikes,  sheets,  etc.,  as  satellites  grouped  about  them. 


DESCRIPTION  OF  IGNEOUS  ROCKS  247 

Perhaps  the  most  notable  instances  of  the  occurrence 
of  these  rocks  are  to  be  found  in  the  great  laccoliths  of  the 
Rocky  Mountains'  region,  in  Colorado,  Utah,  Wyoming 
and  Montana,  which  are  generally  composed  of  granite  or 
syenite  porphyries.  Some  of  these  masses  are 'a  mile  in 
thickness  by  several  in  breadth,  though  often  smaller.  In 
these  regions  they  often  form  powerful  intruded  sheets, 
several  hundred  feet  in  thickness.  It  is  in  the  contact 
zones  of  these  intrusions,  especially  with  limestone,  that 
a  large  proportion  of  the  valuable  ore  deposits,  such  as  the 
silver-lead  ones,  which  have  made  these  regions  famous 
for  their  mining  industries,  are  found.  Thus  to  the 
western  miner  the  word  "  porphyry  "  is  always  of  sugges- 
tive significance. 

Dense  Igneous  Rocks. 

In  the  preceding  groups  of  igneous  rocks  it  is  assumed 
that  all  the  component  grains  of  the  rock,  or  those  forming 
the  greater  part  of  it,  can  be  determined  and  the  mineral 
constitution  safely  established.  In  the  present  group  it 
is  assumed  that  the  texture  of  all  of  the  rock,  or  of  the 
greater  part  of  it,  is  so  dense  that  this  cannot  be  done. 
No  definite  line  can  be  drawn  between  the  two  groups;  in 
many  cases,  whether  a  given  rock  should  belong  to  the  one 
or  the  other,  is  largely  a  matter  of  opinion,  dependent 
upon  the  experience  of  the  observer,  his  knowledge  of 
rocks  and  minerals,  his  power  of  observation,  keenness  of 
eyesight,  and  the  excellence  of  his  lens.  In  this  respect 
we  are  also  limited  by  our  size ;  if  we  were  ants,  instead  of 
men,  who  were  studying  rocks,  it  is  probable  that  few 
would  be  placed  in  this  group. 

As  has  already  been  explained,  under  the  section  treating 
of  the  classification  of  igneous  rocks,  these  dense  varieties 
are  divided  into  two  groups,  on  the  basis  of  color,  into  the 
dark  to  black  basalts  and  the  lighter  colored  felsites;  each 
of  these  has  a  porphyritic  subdivision.  Of  these  the 
felsite  will  be  considered  first. 


248  ROCKS  AND  ROCK  MINERALS 


FELSITE   AND   FELSITE   PORPHYRY. 

The  felsites  include  all  those  dense  igneous  rocks  which 
are  of  stony  texture  and  not  evidently  glassy,  of  all  colors 
except  dark  gray,  dark  green  or  black,  these  latter  belong- 
ing to  basalts.  They  normally  and  commonly  show  light 
shades  of  color;  white,  which  is  not  very  common,  light  to 
medium  gray,  light  pink  or  red  to  dark  red,  pale  yellow  or 
brown,  purple  or  light  green.  With  the  lens  it  can  be  fre- 
quently seen  that  they  consist  of  minute  mineral  grains,  too 
small  for  determination,  and  the  texture  is  then  very  fine 
granular.  In  other  cases  the  grains  may  be  entirely  too 
fine  to  be  seen;  the  rock  has  then  a  dense,  horn-like  or 
flinty  aspect,  appearing  like  a  homogeneous  substance. 
In  this  latter  case  it  is  very  apt  to  have  a  smooth  con- 
choidal  fracture.  In  other  cases,  especially  in  surface 
lavas,  the  texture  is  more  or  less  porous  and  the  fracture 
surface  of  the  rock  rough  and  hackly,  with  a  harsh  feeling. 
A  pronounced  cellular  or  vesicular  structure,  common  in 
basalts  and  in  glassy  rocks,  and  illustrated  on  Plate  8  is  not 
very  common  in  this  group.  The  surface  lavas  not  infre- 
quently show  fluidal  bandings  and  streakings,  more  or  less 
flat  lenticular,  and  often  curved  or  curled,  due  to  flowage, 
and  often  clearly  brought  out  on  weathered  surfaces,  as 
illustrated  on  Plate  22. 

The  division  of  the  dense  igneous  rocks  into  felsites  and  basalts 
is  based  on  color,  and  not  on  mineral  composition,  since  the  latter 
cannot  be  determined.  Nevertheless  the  felsites  as  classed  above 
are,  in  general,  feldspathic  rocks,  and  they  represent  in  dense  form 
and  often  as  lavas  those  magmas,  which  under  different  geologic  and 
physical  conditions,  would  have  produced  granites  and  syenites, 
while  the  basalts  correspond  to  diorites,  gabbros  and  dolerites,  as 
already  explained  under  classification. 

In  many  cases,  where  these  rocks  are  of  medium  gray  or  drab 
color,  it  is  difficult  to  know  whether  to  assign  them  to  the  felsites  or 
basalts.  This  happens  especially  when  they  are  very  dense.  In 
this  case,  if  the  rock  be  sharply  examined  with  a  good  lens,  it  may  be 
seen  that  many  tiny  chips  and  flakes,  only  partly  formed  and  yet  in 
the  main  a  part  of  the  mass,  lie  upon  its  surface,  their  thin  edges 


DESCRIPTION  OF  IGNEOUS  ROCKS  249 

separated  from  it  by  a  flat  underlying  crack.  It  will  be  observed 
that  their  thin  edges  are  very  much  lighter  in  color  than  that  of  the 
rock  upon  which  they  lie  and  are  translucent  to  light;  indeed  in 
many  cases  they  will  appear  practically  white,  even  when  the  rock 
is  a  dark  gray  or  stone  color.  This  is  a  peculiarity  of  those  dense 
rocks  which  are  chiefly  composed  of  feldspar  and  are,  therefore,  to 
be  classed  as  felsites;  it  is  not  shown  at  all,  or  only  in  a  very  small 
degree,  by  the  basalts.  The  reason  for  this  is  that,  although  they 
may  be  colored  by  a  pigment,  the  feldspars  are  transparent  to 
translucent  minerals  and  a  rock  composed  mostly  of  them  is  also 
megascopically  translucent  on  thin  edges,  while  one  composed 
mostly  of  ferromagnesian  mineral  particles  is  not,  since  these  minerals 
are  either  opaque,  or  practically  so,  from  the  megascopic  standpoint. 
The  same  effect  may  be  observed  on  the  very  thin  edge  of  a  flat  chip 
broken  from  the  rock.  In  case  the  rock  is  not  so  dense,  but  that  the 
individual  grains  can  be  seen  with  the  lens,  if  these  are  transparent 
or  translucent  with  light  colors,  it  may  be  assumed  in  general  that 
they  are  mainly  feldspar,  the  ferromagnesian  mineral  grains  being 
mostly  dark,  dull,  to  opaque.  Olivine,  with  its  yellow-green  color, 
however,  is  an  exception  and  must  not  be  confused  with  feldspar. 
It  will  also  be  noted  that  under  a  good  lens  the  mineral  grains,  or 
many  of  them,  appear  much  lighter  in  color  than  does  the  rock  in  its 
general  effect  to  the  eye. 

These  tests,  while  they  cannot  be  held  to  be  exact,  will  of  ten  prove 
of  service  in  helping  to  decide,  in  doubtful  cases,  whether  a  rock 
belongs  to  the  felsites  or  basalts,  for  the  division  between  them, 
while  based  primarily  on  color,  is  due  also  to  a  mineralogical  dif- 
ference as  well. 

It  is  assumed  in  what  has  been  said  regarding  these  rocks  that 
one  is  dealing  with  reasonably  fresh,  unaltered  material,  not  those 
rocks  which  have  been  long  exposed  to  atmospheric  agencies  and 
are  weathered  into  dull  ferrugineous  material,  or  green  masses  of 
chlorite. 

Varieties  of  Felsite.  From  the  megascopic  standpoint 
the  different  varieties  of  felsite  which  can  be  recognized 
are  those  which  are  due  to  color  and  texture  alone;  the 
petrographer  by  the  use  of  the  microscope  on  thin  sections 
is,  however,  able  to  determine  the  different  kinds  of  miner- 
als, which  form  the  minute  grains,  and  to  divide  and  classify 
these  rocks  on  a  mineralogical  basis,  just  as  was  done  with 
the  coarse-grained  rocks,  whose  grains  could  be  seen  and 
determined  by  the  eye. 


250 


ROCKS  AND  ROCK  MINERALS 


This  is  done  on  the  consideration  of  whether  the  rock  contains 
quartz  or  not,  whether  the  predominant  feldspar  is  alkalic  or  a  soda- 
lime  variety,  or  if  it  contains  a  feldspathoid,  such  as  nephelite,  in 
addition  to  the  feldspar.  According  to  this  we  have  the  following 
kinds  recognized  by  petrographers. 


Equivalent  Coarse- 

Equivalent  Coarse 

Chief  Component  Minerals. 

Rock 
Name. 

Grained  Rock  in 
Petrographic 

Rock  in  Field 
Class,  of  this 

Class. 

Book. 

Alkalic  feldspars  and  quartz. 

Rhyolite 

Granite     .    .    . 

Granite. 

Lime-soda    feldspars    and 

Dacite 

Quartz  diorite  . 

Granite. 

quartz. 

Alkalic  feldspars,  little  or 

Trachyte 

Syenite      .    .    . 

Syenite,  mostly. 

no  quartz. 

Soda-lime  feldspars,  little 

Andesite 

Diorite  .... 

Syenite  and  Dio- 

or no  quartz. 

rite. 

Alkalic  feldspars  and  neph- 

Phonolite 

Nephelite  Syenite 

Nephelite  Syenite. 

elite. 

A  variety  containing  lime-soda  feldspars  and  a  feldspathoid  is 
known,  but  is  very  rare.  The  ferromagnesian  minerals  are  present 
in  variable  amounts  and,  according  to  the  dominant  one  of  these,  we 
have  such  terms  as  mica-trachyte,  hornblende-dacite,  augite-andesite, 
etc.,  etc. 

These  are  terms  which  are  constantly  seen  in  geological  literature 
and  in  general  it  can  be  understood  that  the  rock  so  designated  has 
been  subjected  to  microscopic  study.  They  distinguish  varieties  of 
the  felsites  which  cannot  be  accurately  made  without  such  study, 
but  on  the  other  hand  the  following  points  will  serve  in  a  rather 
vague  and  general  way  to  indicate  megascopically,  to  which  of  the 
above  divisions  a  given  felsite  probably  belongs.  If  the  rock  con- 
tains phenocrysts  of  free  quartz,  as  mentioned  later  in  the  descrip- 
tion of  the  porphyritic  varieties  of  felsite,  it  almost  certainly  contains 
a  considerable  amount  of  quartz  in  the  dense  groundmass  and  is 
either  a  rhyolite  or  dacite;  of  these  rhyolite  is  more  common  than 
dacite.  If  the  rock  on  being  tested  yields  gelatinous  silica,  according 
to  the  method  recommended  on  page  115,  it  is  almost  certainly  a 
phonolite.  The  distinction  between  rhyolite  and  dacite,  and  between 
trachyte  and  andesite,  cannot  be  made  megascopically,  since  it 
depends  on  the  determination  of  the  kind  of  feldspar,  and  in  dense 
rocks  this  can  only  be  done  by  optical  means. 


DESCRIPTION  OF  IGNEOUS  ROCKS  251 

Felsite  Porphyry  or  Leucophyre.  While  felsites  occur 
which  contain  no  porphyritical  crystals  it  is  much  more 
common  for  them  to  contain  phenocrysts.  These  may 
be  very  few,  scattered  and  isolated,  or  they  may  be  abun- 
dant. They  may  be  quite  evenly  scattered,  or  they  may 
be  collected  in  groups.  By  increasing  abundance,  when 
they  form  half  the  bulk  of  the  rock  or  more,  they  cause 
transitions  into  the  group  of  determinable  granular  por- 
phyries previously  described.  The  phenocrysts  may  be 
salic,  in  which  case  they  are  quartz  or  feldspar,  or  they  may 
be  ferromagnesian,  biotite,  hornblende  or  pyroxene.  For 
the  appearance  and  properties  of  these  phenocrysts 
reference  may  be  had  to  the  description  of  them  in  the 
former  group  of  porphyries,  page  245. 

In  the  association  of  these  phenocrysts,  while  all  of  the 
above  minerals  occur  at  times  alone,  there  are  apt  to  be 
two  or  more  present.  Quartz  and  feldspar  with  the 
others  is  not  uncommon,  if  a  dark  mineral  is  associated 
with  the  quartz  it  is  most  apt  to  be  biotite ;  hornblende  is 
less  common  and  pyroxene  very  rare.  Feldspar  and  horn- 
blende, pyroxene  and  feldspar  are  very  common.  In 
general  feldspar  is  the  most  common  phenocryst. 

In  naming  these  rocks  the  general  term  of  felsite  por- 
phyry may  be  given  to  them  or  this  may  be  contracted  to 
leucophyre,  from  the  Greek  (A.CVKOS  —  white)  meaning 
light-colored  porphyry.*  If  no  mineral  qualifier  is  used 
with  this  it  is  understood  that  the  phenocrysts  are  of 
feldspar,  since  this  is  most  general.  If  they  are  of  quartz, 
biotite,  hornblende,  etc.,  we  have  as  follows: 

Quartz-felsite-porphyry  or  quartz-leucophyre. 

Hornblende- felsite-porphyry  or  hornblende-leucophyre. 

Biotite-felsite-porphyry  or  biotite-leucophyre. 

Augite-felsite-porphyry  or  augite-leucophyre. 

Hornblende  feldspar-leucophyre. 

Quartz- feldspar-leucophyre. 

*  As  suggested  in  the  "  Quantitative  Classification  of  Igneous 
Rocks."  Messrs.  Cross,  Iddings,  Pirsson  and  Washington,  p.  184. 


252  ROCKS  AND  ROCK  MINERALS 

These  examples  are  sufficient  to  show  how  mineral 
qualifiers  may  be  used  in  designating  the  phenocrysts  of 
these  rocks,  and  how  these  may  be  combined,  if  desired,  to 
give  expression  to  considerable  descriptive  detail.  Other 
examples  will  readily  suggest  themselves. 

The  same  distinctions  are  made  by  petrographers  among  these 
rocks,  as  in  the  felsites  proper,  and  as  described  above.  Thus  we 
find  such  terms  in  use  as  "  rhyolite-porphyry,"  "  mica-andesite- 
porphyry,"  "  augite-trachyte-porphyry,"  etc.,  resulting  from  the 
study  of  thin  sections  and  accurate  determination  of  the  different 
kinds  of  minerals. 

General  Properties.  The  chemical  composition  of  felsites 
is  variable,  depending,  like  the  subvarieties  enumerated 
above,  on  the  prevailing  minerals.  Some  correspond  with 
the  analyses  of  granite  already  given;  these  contain  free 
quartz;  others  are  like  the  analyses  of  syenites  and  such 
must  contain  little  or  no  quartz.  If  the  amount  of  lime 
is  small,  the  silica  below  60  per  cent,  and  the  alkalies  high 
the  rock  must  be  mostly  composed  of  alkalic  feldspar  and 
probably  nephelite  is  present.  The  specific  gravity  ranges 
from  2.4-2.65  and  is  usually  lower  than  that  of  granite 
and  syenite.  The  jointing  is  platy,  or  in  small  blocks; 
columnar  structure  also  occurs,  but  is  not  so  common  nor 
so  perfect  as  in  basalts.  In  normal  weathering  to  soil 
these  rocks  become  discolored,  brownish,  reddish,  etc. 
The  ferromagnesian  mineral  generally  disappears,  leaving 
a  rusty  spot  or  cavity;  the  rock  crumbles  into  debris,  at 
first  largely  through  mechanical  disintegration.  Finally 
the  feldspars  change  to  clay,  as  described  under  granite, 
and  the  change  to  soil  is  complete.  Where  hydrothermal 
action  takes  place,  as  in  the  vicinity  of  ore  veins,  they  are 
often  changed  to  soft  clay-like  masses  consisting  some- 
times of  clays,  and  sometimes  of  sericite,  the  fine  scaly 
form  of  muscovite. 

Occurrence.  Felsites  occur  intrusively  as  dikes  and 
sheets,  and  sometimes  as  the  endomorphic  contact  facies 
of  larger  intrusive  masses,  whose  main  character  is  that  of 
granite,  granite-porphyry,  syenite,  etc.  They  indicate 


DESCRIPTION  OF  IGNEOUS  ROCKS  253 

intrusions  of  magma  into  cold  rocks,  and  when  found  in 
intrusions,  these  are  usually  relatively  small,  or  narrow. 

They  are  much  more  common  extrusively,  as  lava  flows 
and  sheets,  and  they  sometimes  cover  very  large  areas, 
many  hundreds  and  even  thousands  of  square  miles  in 
extent.  In  these  cases,  and  especially  in  volcanic  cones 
or  their  eroded  remnants,  they  are  usually  interbedded 
with  tuffs  and  breccias. 

While  it  would  be  impossible  to  give  any  adequate  list 
of  actual  occurrences  it  may  be  mentioned  that  felsites  as 
intrusives  occur  extensively  in  eastern  North  America, 
among  the  older  rocks  in  the  Atlantic  border  states,  along 
the  eastern  front  of  the  Appalachian  uplift;  as  extrusives 
they  occur  in  Maine,  in  the  White  Mountains,  in  Penn- 
sylvania and  to  the  southward.  They  occur  also  in 
Wisconsin.  Much  more  extensive  in  western  America  is 
their  effusive  occurrence  throughout  the  whole  of  the 
Cordilleran  tract,  where  they  play  an  important  r61e  in  the 
upbuilding  of  many  of  the  ranges,  and  sometimes  occupy 
large  areas.  Here  all  the  different  varieties  are  found,  as 
for  instance  rhyolite  in  the  Yellowstone  Park,  in  Colorado 
and  elsewhere;  andesite  in  the  lofty  volcanoes  of  the 
Coast  Ranges,  and  in  those  in  South  America,  and  in  the 
eroded  ones  of  the  main  chain  of  the  Rocky  Mountains; 
the  rare  variety  phonolite  is  found  at  Cripple  Creek, 
Colorado,  and  in  the  Black  Hills  of  South  Dakota. 

The  felsites  are  just  as  extensively  found  in  other  parts 
of  the  world.  In  Europe  they  are  well  distributed  as 
effusive  lavas.  So  in  Great  Britain  they  occur  in  northern 
Wales,  in  the  Lake  district  of  England,  and  in  northern 
Ireland.  These  are  the  varieties  called  rhyolite  and 
andesite.  They  are  found  in  western  Germany,  in  France, 
Italy,  and  in  Hungary.  Wherever  volcanic  outbreaks 
have  taken  place  extensively,  felsite  lavas  occur. 

Felsites  and  felsite  porphyries  are  often  found,  in  the  form  of 
narrow  dikes  and  sheets,  traversing  larger  stocks  or  intrusions  of 
coarse-grained  rocks,  such  as  granite,  syenite,  etc.,  or  the  rocks 


254        ROCKS  AND  ROCK  MINERALS 

in  their  immediate  vicinity.  They  are  generally  complementary  in 
this  case  to  dark  basaltic  dikes,  mentioned  later  as  lamprophyres, 
and  have  received  special  names  from  petrographers.  Thus  we 
have  bostonite  (allied  to  trachyte),  tinguaite  (allied  to  phonolite)  and 
many  others.  Note  in  this  connection  paragraphs  on  complemen- 
tary rocks,  on  aplite,  and  on  lamprophyres. 

BASALT  AND   BASALT-PORPHYRY. 

The  basalts  include  those  dense  igneous  rocks,  of  very 
dark  color,  whose  fabric  is  so  fine  that  the  constituent 
grains  either  cannot  be  perceived  by  the  eye  or  with  the 
lens,  or,  if  seen,  are  too  small  to  be  recognized,  and  which 
are  of  stony  but  not  of  glassy  texture.  The  color  varies 
from  grayish  black  or  dark  stone  color,  greenish  or  purplish 
black,  to  pure  black.  In  the  great  majority  of  cases  they 
do  not  show  translucency  on  the  edges  of  thin  flakes  as 
described  under  felsite.  When  not  cellular,  and  very 
dense,  they  have  a  uniform  dull,  soft,  almost  velvety 
appearance,  and  do  not  show  the  horny,  flinty,  or  greasy 
luster  of  many  dense  felsites. 

The  study  of  thin  sections  of  these  rocks  shows  that  in  general 
they  are  composed  of  minute  crystal  grains  of  soda-lime  feldspar  — 
generally  labradorite,  —  pyroxene  and  iron  ore,  very  often  with 
more  or  less  olivine,  and  sometimes  biotite  or  hornblende.  In  some 
cases'  nephelite  or  leucite  may  accompany  the  feldspar,  or  replace  it, 
giving  rise  to  varieties  which  have  received  special  names.  These 
varieties,  although  very  interesting  from  the  standpoint  of  theoretical 
petrography,  are  comparatively  rare  and  relatively  of  small  impor- 
tance in  a  general  geological  way. 

Being  composed  of  the  same  minerals,  these  rocks  represent,  in 
dense  form  and  generally  as  surface  lavas,  those  magmas  which, 
under  different  physical  conditions,  would  have  solidified  as  gabbros, 
peridotes,  dolerites  and  (in  part)  diorites.  A  large  part  of  the 
dolerites  in  fact  are  transition  rocks  between  them  and  gabbros,  as 
previously  mentioned  in  the  description  of  that  rock,  and  instances 
may  be  found  in  the  same  rock  mass  where  the  gradation  into 
dolerite  may  be  seen. 

Basalt  Porphyry.  While  porphyritic  varieties  of  basalt 
are  not  uncommon  rocks  it  may  be  said  in  general,  that  this 
type  of  texture  plays  a  far  less  important  role  in  this 


PLATE  20. 


A.    Labradorite-Porphyry. 


B.    Augitophyre. 
VARIETIES  OF   BASALT-PORPHYRY. 


DESCRIPTION  OF  IGNEOUS  ROCKS  255 

group  than  in  the  felsites  previously  described,  owing 
probably  to  the  low  freezing  point  and  easy  crystallization 
of  the  magma.  One  exception  to  this,  however,  is  in  the 
presence  of  olivine,  which  is  very  apt  to  occur  scattered 
through  the  basalt  in  transparent  yellowish  or  bottle- 
green  porphyritic  grains,  averaging  about  the  size  of 
moderately  coarse  shot.  The  mineral  is  so  common 
(indeed  at  one  time  a  rock  was  not  considered  a  basalt 
unless  it  contained  olivine)  and  produces  in  general  so 
little  of  a  striking  porphyritic  effect,  that  it  is  rather  the 
custom  to  ignore  it  in  this  respect,  and  term  such  rocks 
oli vine-basalt  rather  than  olivine-basalt-porphyry. 

The  chief  minerals  as  phenocrysts,  when  such  occur,  are  feldspar 
and  pyroxene;  hornblende  and  mica  are  much  less  common.  The 
feldspar  is  commonly  labradorite;  it  occurs  in  elongate  tabular  forms, 
either  singly,  or  in  twinned  groups.  The  pyroxene  is  the  variety 
augite ;  it  is  black,  sometimes  shining,  sometimes  dull,  and  is  in  short 
thick  prisms  or  prismoids,  as  illustrated  under  pyroxene.  The 
hornblende  is  also  black  and  has  its  usual  shining  and  good  cleavage. 
Biotite  is  in  six-sided  tablets. 

These  rocks  would  be  named  in  accordance  with  the 
prevailing  phenocryst,  so  for  example  augite-basalt- 
porphyry.  Instead  of  the  term  basalt-porphyry  the  name 
melaphyre,  meaning  "  black  porphyry,"  may  be  used  as 
more  convenient  *  and  we  should  then  have  feldspar- 
melaphyre,  augite-melaphyre,  biotite-melaphyre,  etc. 

General  Properties  of  Basalt.  The  chemical  composition 
of  basalt  varies  with  its  mineral  composition;  in  general 
it  is  of  the  same  nature  as  that  of  gabbro  previously  given, 
as  may  be  seen  from  the  following  analysis  of  one  from 
California,  which  will  serve  as  an  example. 

SiO2  A12O3  Fe2O3  FeO  MgO  CaO  Na2O  K2O  H2O  XyO  Total 
51.9     15.3       3.1      3.6     8.7      7.4      3.3      2.5      2.5     1.7  =  100.0 

The  specific  gravity  is  high,  about  3.0  (2.9-3.1).  The 
•jointing  is  platy  or  columnar;  the  best  examples  indeed  of 

*  Quantitative  Classification  of  Igneous  Rocks,  p.  185 


256        ROCKS  AND  ROCK  MINERALS 

this  structure  are  found  in  basalt  and  many  notable 
examples  of  it  are  found  in  all  parts  of  the  world,  the 
Giants'  Causeway  on  the  north  coast  of  Ireland  being  one 
of  the  best  known.  This  structure  is  seen  in  Plate  11. 
Sometimes  basalt  on  weathering  develops  a  singular  "pil- 
low" structure  by  which  there  is  formed  spheroidal  masses. 

Varieties.  In  the  dense  non-porphyritic  basalts  there 
is  little  opportunity  for  variation,  save  that  which  is  based 
on  a  change  from  the  compact  into  the  porous  or  cellular 
structure.  This  last  is  particularly  common  in  surface 
lavas,  especially  in  their  upper  portion,  and  has  been 
illustrated  on  Plate  8.  It  is  particularly  in  these  basalts 
that  the  amygdaloidal  structure  occurs,  also  illustrated  on 
the  same  plate.  The  minerals  filling  the  cavities  in 
basalt  are  commonly  quartz,  calcite  or  zeolites;  among 
the  latter  minerals  analcite,  natrolite,  stilbite  and  heu- 
landite  may  be  particularly  mentioned.  Such  rocks  are 
termed  amygdaloidal  basalt.  In  a  number  of  places,  and 
particularly  in  western  America,  basalts  have  been  found 
as  surface  lavas  which  contain  visible  grains  of  quartz. 
One  of  the  most  noted  of  these  is  the  basalt  flow  from  the 
Cinder  Cone,  near  Lassens  Peak  in  northern  California, 
which  is  filled  with  angular  pieces  of  quartz  of  varying 
sizes.  As  many  of  these  correspond  in  composition  to 
gabbros  and  dolerites,  the  presence  of  the  quartz  in  them 
appears  anomalous,  since  magmas  so  low  in  silica,  as  may 
be  seen  by  referring  to  the  analyses  of  gabbro,  would  not 
be  expected  to  develop  free  quartz  on  crystallizing.  Some 
petrographers  therefore  think  that  these  are  fragments  of 
quartz  rock  in  the  depths,  which  have  been  torn  loose  and 
distributed  through  the  magma,  while  others  regard  them 
as  a  primary  crystallization,  produced  under  exceptional 
conditions  of  pressure  and  mineralizers.  These  rocks 
have  been  called  quartz-basalts. 

The  porphyritic  varieties  have  been  described  above, 
but  it  may  be  mentioned  that  a  variety  containing  distinct 
and  sometimes  large  crystals  of  labradorite  feldspar  has 


DESCRIPTION  OF  IGNEOUS  ROCKS  257 

been  called  labradorite-porphyry.  The  greenish-black  por- 
phyry from  Greece,  employed  by  the  ancients  (porfido 
verde  antico),  is  a  somewhat  altered  example  of  this  type. 
A  variety  containing  rather  large  and  distinct  crystals  of 
augite  has  been  termed  augitophyre. 

The  name  trap  has  been  used  in  a  general  way  as  a  field 
term  to  designate  the  rocks  called  here  basalts,  and  also 
dolerites.  As  thus  employed  it  would  mean  any  dark- 
colored,  heavy,  igneous  rock  of  undetermined  mineral 
composition.  Thus  the  dolerites  and  basalts  of  the 
Newark  formation  along  the  Atlantic  coast  have  been 
termed  "  Triassic  traps;"  the  great  effusives  of  western 
India  are  known  as  the  "  Deccan  traps." 

Lamprophyres.  The  ferromagnesian  complementary 
rocks,  occurring  in  dikes  and  sheets  in  or  around  stocks 
of  granite,  syenite,  etc.,  and  often  called  "  trap  "  dikes, 
etc.,  belong  for  the  most  part  under  this  heading  of 
basalt.  Their  origin  and  relations  have  been  discussed 
in  the  preceding  part  of  this  work  and  they  have  been 
mentioned  again  under  granite.  They  are  very  apt  to 
contain  phenocrysts  of  the  ferromagnesian  minerals, 
olivine,  augite,  hornblende  and  biotite,  either  separately 
or  together,  and  sometimes  these  phenocrysts  are  of  very 
large  size.  These  are  embedded  in  a  groundmass  that  is 
usually  dense  and  basaltic.  According  to  the  variations 
in  the  minerals,  as  shown  by  the  microscope,  a  large 
number  of  different  types  have  been  named  by  petrogra- 
phers,  distinctions  which  ordinarily  cannot  be  made  mega- 
scopically.  For  field  work  they  may  be  treated  simply  as 
basalt-porphyries  as  described  above,  and  termed  augite 
melaphyre,  biotite  melaphyre,  etc.  It  may  be  mentioned 
that  biotite  melaphyre  is  a  rock  which  is  frequently  found 
in  dikes  with  granite,  and  has  been  called  mica  trap  or 
minette.  A  hornblende  melaphyre  occurs  in  the  same  way 
with  many  syenites  and  nephelite  syenites;  it  has  quite 
a  wide  distribution  in  New  England  and  has  been  termed 
camptonite. 


258  ROCKS  AND  ROCK  MINERALS 

Olivlne  Nodules.  It  frequently  happens  that  basalts,  in 
addition  to  the  ordinary  crystals  of  oh'vine,  contain  yellow- 
ish, or  green  lumps,  or  nodules,  made  up  of  grains  of  this 
mineral.  Grains  of  other  minerals,  such  as  pyroxene, 
spinel,  etc.,  may  be  present  in  them.  These  lumps  may 
vary  in  size  from  a  pea  to  masses  as  large  as  one's  fist, 
or  even  larger.  They  are  generally  rounded,  but  often 
distinctly  angular  in  shape.  Their  origin  is  somewhat 
problematical;  some  hold  that  they  are  merely  agglomera- 
tions of  the  earlier  formed  crystals  in  the  liquid  magma, 
while  others  regard  them  as  fragments  of  rock  (dunite) 
torn  off  below  and  brought  up  in  it. 

Exotic  Minerals.  Basalts  sometimes  contain  unusual 
minerals,  which  do  not  appear  in  the  ordinary  rock,  and 
whose  origin  in  them  must  be  ascribed  to  unusual  condi- 
tions, or  composition  of  the  magma.  The  quartz  basalt 
mentioned  above  is  one  of  these.  Another  case  is  seen  in 
the  iron-bearing  basalts  of  Greenland,  which  contain 
small  to  large  masses  of  native  iron,  which  is  much  like 
the  iron  found  in  meteorites.  By  the  use  of  a  solution  of 
copper  sulphate  specks  of  native  iron  have  been  found  in 
basalts  from  other  places.  The  Greenland  basalts  also 
contain  graphite.  Corundum,  in  the  form  of  sapphire,  has 
also  been  found  in  basalts,  and  a  dike  in  Montana  has 
furnished  a  quantity  of  valuable  gems.  In  this  connection 
also,  may  be  mentioned  the  occurrence  in  places  of  native 
copper,  especially  in  the  Lake  Superior  district,  where  the 
metal  occurs  in  dolerites  and  basalts  and  in  connection 
with  them,  in  quantities  which  have  made  it  one  of  the 
most  important  sources  of  the  world's  copper  supply. 

Weathering  and  Alteration.  In  many  volcanic  regions, 
where  basalts  have  been  subjected  to  exhalations  of  steam 
or  to  heated  water,  the  minerals  containing  ferrous  oxide, 
such  as  magnetite  and  olivine,  become  reddened  through 
change  to  ferric  oxide.  Sometimes  the  olivines  alone  are 
reddened;  in  other  cases  the  whole  rock  becomes  deep  red 
to  reddish  brown.  Such  rocks  may  be  difficult  to  dis- 


DESCRIPTION  OF  IGNEOUS  ROCKS  259 

tinguish  in  the  field  from  red  felsites,  and  may  even  have 
to  be  classed  with  them.  Sometimes,  however,  the  asso- 
ciation with  other  rocks,  the  retained  form  of  pheno- 
crysts,  and  the  good  amygdaloidal  structure,  rarely  seen 
in  the  most  common  felsites,  may  help  one  to  recognize 
the  original  character  of  the  rock. 

The  normal  weathering  of  basalt  gives  rise  to  chlorite, 
serpentine,  and  carbonates,  with  clay  and  iron  ores;  the 
rock  often  turns  green  and  becomes  soft  when  much 
chlorite  is  developed.  In  other  cases  it  turns  brown 
through  oxidation  and  eventually  falls  away  into  brownish 
ferrugineous  soil,  to  which  various  names  are  given,  as 
laterite  in  India,  wacke  in  Germany,  etc.  Sometimes  from 
such  deposits  all  but  the  hydroxides  of  iron  and  alumina 
are  leached,  forming  one  variety  of  the  so-called  beauxite. 

Under  processes  of  metamorphism  the  basalts  act  like 
the  gabbros  and  dolerites  previously  described,  and  give 
rise  to  "  greenstone "  and  to  greenstone  schists  and 
amphibolite. 

Occurrence  of  Basalt.  As  intrusive  rocks,  sheets,  and 
especially  dikes,  of  basalt  of  various  types,  both  plain  and 
porphyritic  in  texture,  are  so  common  in  all  regions  where 
igneous  rocks  occur  that  they  need  no  further  mention. 
As  extrusive  lavas,  in  the  form  of  flows  and  extended 
sheets,  they  are  of  much  greater  geological  interest  and 
importance.  There  is  scarcely  any  volcanic  region  in  the 
world  which  does  not  exhibit  them  in  greater  or  lesser 
amount,  and  in  some  regions,  as  in  the  lava  fields  of  the 
Columbia  in  western  America,  and  in  western  India,  they 
have  been  poured  out  in  stupendous  masses,  so  that  tracts 
of  country  nearly  200,000  square  miles  in  extent  have 
been  covered  thousands  of  feet  deep.  A  similar  great 
field  existed  in  northern  Great  Britain,  and  its  remnants, 
portions  yet  saved  from  the  eroding  edge  of  the  Atlantic, 
form  in  great  part  the  northern  British  Isles. 

Leuclte  Rocks.  Basaltic  rocks  in  which  the  feldspathoid  minerals, 
nephelite  or  leucite,  are  present,  either  accompanying  the  feldspar 


260  ROCKS  AND  ROCK  MINERALS 

or  replacing  it,  while  not  common,  have  in  certain  regions  a  consider- 
able local  development.  Ordinarily  these  minerals  are  in  the 
groundmass,  and  only  to  be  detected  by  the  microscope,  and  such 
rocks  in  the  field  must  be  classed  as  regular  basalts.  In  central 
Italy,  however,  the  leucite  rocks  have  a  great  development,  and  in 
many  cases  the  leucite  crystals  appear  as  phenocrysts  as  large 
as  peas,  or  larger  at  times,  and  are  easily  recognized.  They  are 
leucite-basalt-porphyries  or  leucite-melaphyre.  For  the  properties 
of  leucite  its  description  under  rock  minerals  should  be  con- 
sulted. According  to  the  other  minerals  present,  several  different 
types  of  these  rocks  are  distinguished  and  named.  Some  of  them 
are  so  light  colored  they  would  be  classed  as  varieties  of  felsites. 
Outside  of  Italy  these  leucite  rocks  are  very  rare,  occurrences  being 
known  in  the  Rhine  district,  in  central  Montana,  western  Wyoming 
and  a  few  other  localities,  but  since  the  well-known  lavas  of  Vesuvius 
are  composed  of  them,  they  are  mentioned  here. 

Glassy  Rocks. 

In  the  felsites  and  basalts  the  use  of  the  microscope  on 
thin  sections  would  show  in  many  cases  that  a  certain 
amount  of  glass,  uncrystallized  and  solidified  magma,  is 
present  in  them,  acting  as  a  cement  to  hold  the  mineral 
grains  together.  This  cannot  be  detected  megascopically, 
and  under  the  term  of  glassy  rocks,  as  here  used,  is  meant 
only  such  as  are  entirely  of  glass,  or  if  partly  crystalline, 
those  containing  it  in  such  amounts  and  in  such  circum- 
stances, that  it  is  visible  and  evident  to  the  eye. 

The  conditions  which  will  cause  a  magma  to  solidify  as 
a  glass  are  evidently  those  which  are  unfavorable  to  crys- 
tallization, extremely  quick  cooling  in  the  first  place,  and 
probably  to  some  extent  the  rapid  loss  of  mineralizers  in 
the  second.  This  has  been  already  discussed  in  connection 
with  the  texture  of  igneous  rocks.  These  conditions  are 
best  realized  when  the  magmas  are  poured  out  on  the 
surface  as  effusive  lavas,  and  just  as  we  associate  a 
coarse-textured,  entirely  crystalline,  granular  rock,  such 
as  granite,  with  an  intrusive  or  deeply  seated  origin,  so 
conversely  we  associate  glassy  rocks  with  an  extrusive 
one.  Indeed  while  it  is  true  that  dikes  may  sometimes 


DESCRIPTION  OF  IGNEOUS  ROCKS  261 

show  glassy  selvages  along  the  contact,  when  they  have 
been  intruded  into  cold  rocks,  or  may  indeed  be  wholly 
of  glass  when  the  exposure  is  near  the  original  surface,  as 
in  recently  denuded  dikes  in  volcanic  regions,  this  is  so 
uncommon  and  so  inconsiderable  an  affair,  that  in  general 
we  may  regard  the  fact,  that  a  rock  is  composed  partly  or 
wholly  of  evident  glass,  as  a  proof  of  its  extrusive  origin, 
that  it  was  originally  a  surface  lava,  although  it  may  have 
been  buried  under  later  formations. 

Any  of  the  different  magmas,  varying  as  to  composition, 
may  form  glassy  rocks  if  chilled  with  sufficient  rapidity, 
but  petrographical  research  has  shown  that,  while  glassy 
forms  of  the  felsite  lavas  are  common,  those  corresponding 
to  basalt  are  much  rarer  and  relatively  of  inconsiderable 
volume.  The  reason  for  this  appears  to  be  that  the  mag- 
mas, which  furnish  felsites,  or  the  granite  and  syenite  which 
correspond  to  them,  have  a  relatively  high  freezing  point, 
and  as  the  magma  cools  down  and  approaches  this,  it 
becomes  so  enormously  viscous  that  the  free  movement 
of  molecules  necessary  for  crystallization  is  prevented. 
This  is  due  to  the  large  amount  of  silica  that  such  magmas 
contain,  which  has  a  strong  effect  in  promoting  viscosity. 
The  presence  of  water  in  the  magma  tends  to  neutralize 
this,  and  to  make  the  magma  more  fluid  and  thus  to  help 
crystallization,  but  when  it  is  poured  out  on  the  surface 
the  water  is  rapidly  lost  with  increase  in  viscosity.  On 
the  other  hand  the  basaltic  magmas,  or  those  corresponding 
to  gabbro  or  diorite  in  part,  which  contain  relatively 
low  silica  and  high  iron  and  magnesia,  have  a  much  lower 
freezing  point  and  remain  liquid  as  they  approach  it, 
thus  permitting  easy  crystallization  and  the  assumption 
of  stony  texture  and  appearance.  Consequently  those 
glasses,  which  have  the  highest  percentage  of  silica  and 
correspond  to  granite  in  composition,  are  the  most  common 
ones. 

Classification  of  Glassy  Bocks.  As  already  stated  in 
the  classification  of  igneous  rocks,  we  may  divide  the 


262  ROCKS  AND  ROCK  MINERALS 

glassy  rocks  into  two  groups,  one  containing  distinct 
crystals  or  phenocrysts  embedded  in  a  glass  base,  or 
porphyritic  varieties  in  short,  and  second,  those  without 
distinct  phenocrysts,  consisting  of  either  pure  glass,  or 
glass  more  or  less  filled  with  spherulites  or  lithophysae,  as 
described  later.  The  second  group  is  again  subdivided 
according  to  luster  and  structure.  In  accordance  with 
this  we  have  as  follows: 

(Obsidian,  strong  bright  vitreous  luster. 

Glass  with  few  or  nol  Pitchstone,  dull  pitchy  or  resinous  luster, 
phenocrysts  j  Perlite,  apparently  made  of  small  spheroids . 

I  Pumice,  cellular  structure,  glass  froth. 
Glass    more    or    less  r 

filled    with    pheno- J  Vitrophyre,  glass  porphyry, 
crysts  [ 

Obsidian.  This  is  pure,  solid,  natural  glass,  devoid  of 
all  apparent  crystal  grains,  or  nearly  so.  It  has  a  bright 
luster  like  that  of  artificial  glass.  It  usually  has  a  jet- 
black  color,  but  when  the  edges  of  thin  chips  are  examined 
against  the  light  it  is  generally  seen  to  be  transparent  or 
translucent  with  a  more  or  less  smoky  color,  and  it  can  be 
often  observed  with  a  lens  that  the  coloring  matter  is 
more  or  less  collected  into  fine  parallel  streaks,  bands,  or 
threads,  as  if  drawn  out  in  the  flowage.  Less  commonly 
the  glass  is  gray,  or  Indian  red,  or  rich  brown,  and  this  is 
sometimes  mixed  with  the  black  in  bands  and  strings, 
which  kneaded  through  it  produce  a  marbled  effect.  The 
microscope  shows  the  black  glass  as  colorless  and  filled 
with  tiny,  black,  dust-like  particles;  they  are  probably 
specks  of  magnetite,  which  represent  the  beginnings  of 
crystallization,  and  diffused  through  the  glass,  they  act  as 
a  pigment,  coloring  it  black.  In  other  cases  they  have 
been  oxidized  to  hematite  dust  and  the  color  is  then  red 
or  brown. 

Obsidian  has  a  remarkable  conchoidal  fracture,  illus- 
trated in  Figure  4,  page  29,  due  to  its  homogeneity  and 
lack  of  structure.  It  was  this  quality  that  made  the 


DESCRIPTION  OF  IGNEOUS  ROCKS 


263 


substance  so  highly  valued  by  primitive  peoples,  for  it 
enabled  them  by  chipping  to  work  it  into  desired  forms, 
knives,  spearheads  and  other  implements  and  weapons, 
while  long,  slender  flakes  possessed,  for  cutting  pur- 
poses, knife-edges  of  razor-like  keenness.  The  ancient 
Mexicans  were  especially  skilful  in  working  it,  and  were 
able  to  spring  off  blades  of  bayonet-like  cross-section, 
half  an  inch  in  breadth  by  six  inches  or  more  in  length. 

While  obsidian  corresponding  to  the  various  kinds  of 
igneous  rocks  is  known,  it  usually  has  a  composition 
similar  to  that  of  granite,  as  may  be  seen  from  the  analysis 
of  a  typical  specimen  from  the  Yellowstone  Park. 


Si02 

A1203 

Fe203 

FeO 

MgO 

CaO 

Na2O 

KjO 

H2O 

FeS2 

74.7 

13.7 

1.0 

0.6 

0.1 

0.8 

3.9 

4.0 

0.6 

0.4 

=99.8 

It  can  be  readily  shown  by  calculation  that  had  this 
magma  crystallized,  it  would  have  produced  a  rock  con- 
sisting of  35  per  cent  of  quartz,  60  per  cent  of  feldspar, 
with  5  per  cent  of  other  minerals,  that  is  to  say,  a  granite. 
The  specific  gravity  varies  from  2.3-2.7,  depending  on  the 
composition;  of  the  most  common  variety,  2.3-2.4.  The 
hardness  is  greater  than  that  of  ordinary  window  glass, 
which  it  scratches.  Before  the  blowpipe  a  splinter  of 
black  obsidian  fuses  readily,  with  bubbling,  to  a  vesicular 
gray  or  white  enamel,  which,  after  the  removal  of  the 
water,  becomes  exceedingly  infusible.  This  experiment 
is  very  instructive  in  showing  the  effect  of  water  in  lower- 
ing the  fusing  point  of  magmas  and  in  increasing  their 
liquidity.  The  water  in  the  obsidian  is  not  the  product 
of  alteration,  for  it  is  present  in  what  the  microscope 
reveals  as  the  purest  and  clearest  glass,  nor  are  there 
cavities  to  contain  it;  it  appears  to  be  chemically  a  part  of 
the  mixture,  like  Na2O  and  K2O. 


264  ROCKS  AND  ROCK  MINERALS 

Spherulites.  In  many  obsidians  may  be  seen  rounded, 
sometimes  perfectly  spherical,  bodies  of  white,  gray  or  red 
color,  varying  in  size  from  those  of  microscopic  dimen- 
sions up  to  those  of  an  egg,  or  even  larger;  usually  from 
the  size  of  fine  shot  to  peas.  If  closely  examined  with  a 
lens,  it  can  generally  be  seen  that  they  are  composed  of 
fibers  radiating  from  a  common  center;  at  uniform  dis- 
tances from  the  center  the  fibers  are  apt  to  change  color, 
or  to  be  saturated  with  a  differently  colored  material,  and 
the  body  appears  built  of  successive  concentric  shells. 
These  bodies  are  called  spherulites  and  are  composed  of 
fibers  of  feldspar.  They  are  indicative  of  sudden  cooling 
and  a  very  rapidly  induced  crystallization,  the  fibers 
shooting  outward  from  some  center  where  crystallization 
commences,  and  branching  as  they  grow,  until  checked  by 
the  viscosity  of  the  rapidly  cooling  magma.  They  should 
not  be  confused  with  phenocrysts  which  are  single, 
individual  crystals.  An  example  is  shown  on  Plate  21. 
They  are  sometimes  formed  by  accident  in  artificial  glass, 
as  seen  on  Plate  21;  in  this  case  the  artificial  mineral 
forming  them  is  wollastonite,  CaSiOa. 

Frequently  the  spherulites  form  before  the  lava  has 
come  to  rest  and  are  thus  drawn  out,  so  that  they  are 
dotted  along  the  rock  in  lines.  When  in  great  num- 
bers, and  minute,  they  may  coalesce;  some  streaks  of  the 
rock  are  then  composed  of  them,  while  other  bands  are 
of  dark,  solid  glass,  as  shown  in  Plate  22. 

Lithophysae.  Closely  connected  with  the  spherulites 
there  occur  also  in  glassy  rocks  peculiar  formations  known 
as  lithophysae  (stone  bubbles).  These  consist  of  a  series 
of  concentric  shells  of  crystalline  material,  resembling  some- 
what nested  watchglasses,  which  surround  a  central  cavity, 
and  are  more  or  less  separated  from  each  other.  They 
consist  of  adherent  crystals,  and  are  very  fragile.  When 
exposed  by  the  breaking  of  the  rock  they  appear  much 
like  flowers  with  concentric  layers  of  petals.  They  vary 
in  size  from  very  small  to  several  inches  in  diameter.  The 


PLATE  21. 


A.    Spherulites  in  Obsidian. 


B.    Spherulites  in  Glass. 


C.    Lithophysae.  D.    Lithophysae. 

STRUCTURES  IN   GLASSY    ROCKS. 


DESCRIPTION  OF  IGNEOUS  ROCKS  265 

walls  of  the  cavities  are  coated  with  minute  but  beautiful 
crystals  of  quartz,  tridymite  and  feldspar,  and  sometimes 
fayalite,  topaz,  garnet  and  tourmaline  are  found  in  them. 
Sometimes  they  are  more  or  less  flattened  and  strung 
along  the  flowage  planes  of  the  rock.  They  occur,  not 
only  in  the  pure  glassy  lavas,  but  also  in  those  which  by 
more  or  less  crystallization  have  assumed  megascopically 
a  stony  texture  and  appearance.  They  are  illustrated  on 
Plate  21.  Their  origin  is  ascribed  to  repeated  shells  of 
crystallization,  with  consequent  liberation  of  water  vapor, 
and  expansions  of  the  cavities  through  its  influence  under 
high  temperature.  The  formation  of  topaz  and  other 
minerals  points  to  the  presence  of  fluorine  and  other 
accompanying  gases.  Thus  the  lithophysae  seem  to 
bear  a  certain  analogy  to  miarolitic  cavities  in  the  intrusive 
rocks,  as  described  elsewhere. 

Pitchstone.  This  may  be  regarded  as  merely  a  variety 
of  obsidian  in  which  the  luster,  instead  of  being  bright  and 
glassy,  is  duller  and  the  rock  appears  resinous  or  pitchlike. 
There  is  also  a  chemical  difference  in  that,  while  the 
water  contained  in  obsidian  is  rarely  so  much  as  one 
per  cent  and  may  sink  to  mere  traces,  pitchstone  contains 
much  more,  as  much  as  5  or  6  per  cent  or  even  greater. 
It  is  this  which  probably  influences  the  luster.  They  are 
also  variable  in  color,  —  black,  gray,  red,  brown,  and  green, 
and  are  translucent  to  transparent  on  thin  edges. 

Perlite.  This  is  a  peculiar  variety  of  glassy  rock  which 
is  composed  of  small  spheroids,  usually  varying  in  size 
from  small  shot  to  peas.  It  is  generally  of  a  gray  to  blue- 
gray  color,  rarely  red,  has  a  soft,  pearly,  or  wax-like  luster 
and  resembles  enamel.  The  spheroids  either  lie  separated 
in  a  sort  of  cement  and  are  then  round,  or  they  may  be 
closely  compressed  and  are  then  polygonal.  They  tend 
to  have  a  concentric,  shelly  structure  and  are  the  result  of 
a  contraction  phenomenon  in  the  cooling  glass,  which  pro- 
duces a  spherical,  spiral  cracking,  as  shown  in  thin  sections. 
Analyses  of  perlites  prove  them  to  have  a  rather  constant 


266  ROCKS  AND  ROCK  MINERALS 

percentage  of  combined  water,  between  3  and  4  per  cent, 
and  there  may  be  a  connection  between  this  amount  of 
water  and  the  peculiar  method  of  cracking.  To  the  casual 
observer  they  somewhat  resemble  oolites  and  pisolites  of 
the  concretionary  sedimentary  rocks.  Perlite  is  produced 
only  by  felsitic  magmas,  especially  by  those  high  in  silica; 
it  does  not  occur  in  basaltic  glasses. 

Pumice,  Scoria,  etc.  Pumice  is  highly  vesicular  glass 
produced  by  the  extravasation  of  the  water  vapor  at  high 
temperature,  through  relief  of  pressure,  as  the  magma  comes 
to  the  surface.  It  is  best  described  as  glass  froth.  Its 
color  is  white,  gray,  yellowish,  or  brownish,  rarely  red.  It 
sometimes  has  a  somewhat  silky  luster.  Examined  with 
the  lens  it  is  seen  to  be  composed  of  a  mass  of  silky  glass 
fibers  of  a  cottony  appearance,  full  of  pores,  and  separated 
by  larger  holes  like  a  sponge.  If  drawn  out  by  flowage 
the  fibers  are  parallel,  otherwise  they  are  interwound. 
The  chemical  composition  of  typical  pumice  is  like  that 
of  the  highly  siliceous  obsidian,  or  in  other  words  like 
that  of  granite.  Pumice  does  not  form  independent  rock- 
masses,  it  occurs  as  the  upper  crust  of  flows  of  felsite  lava, 
or  in  fragments  among  the  explosive  material  ejected  by 
volcanoes.  On  account  of  its  light,  porous  nature,  and  its 
content  of  sealed  glass  cells,  it  floats  almost  indefinitely 
on  water,  and  the  material  ejected  by  volcanoes  near  or 
in  the  sea  is  borne  by  currents  all  over  the  world,  and 
drifts  ashore  everywhere.  Its  use  as  an  abrasive  and 
polishing  agent,  and  for  toilet  purposes,  is  due  to  the  sharp 
cutting  edges  of  the  thin  films  and  fibers  of  glass;  nearly 
all  that  is  used  comes  from  the  Lipari  Islands  off  the  coast 
of  Sicily.  Other  places  of  occurrence  are  mentioned 
later. 

Scoria.  While  all  magmas,  whatever  their  chemical 
composition,  at  times  and  under  proper  conditions,  form 
pumiceous  rocks,  typical  pumice,  as  stated  above,  is  most 
characteristic  of  the  felsitic  ones,  while  basaltic  pumices 
are  of  local  development  and  of  inconsiderable  impor- 


PLATE  22. 


A.   FLOW   STRUCTURE   IN   GLASSY   LAVA. 


B-  SLAGGY   STRUCTURE   OF   BASALTIC   LAVA 


DESCRIPTION  OF  IGNEOUS  ROCKS  267 

tance.  Nevertheless  the  basaltic  magmas  develop  through 
the  expansion  of  gases  vesicular  forms,  as  described  under 
basalt.  These  pass,  especially  on  the  upper  surface  of 
basalt  flows,  and  in  the  material  thrown  out  by  volcanoes, 
into  more  or  less  glassy,  partly  stony,  dark  or  reddish, 
loosely  compacted,  spongy,  cindery  or  slag-like  modifica- 
tions known  as  volcanic  scoria.  This  form  is  illustrated 
on  Plate  23. 

A  peculiar  modification  of  what  may  be  considered  basaltic 
pumice  occurs  in  the  crater  of  Kilauea  in  Hawaii,  where  drops  of 
lava  flying  up  from  the  boiling  lava  lakes  pull  out  thin,  hair-like 
threads  of  glass  after  them.  These  threads,  drifted  by  the  wind, 
collect  in  tow-like  masses,  called  by  the  natives  "  Pele's  hair  "  after 
the  titulary  goddess  of  the  islands. 

Vitrophyre.  Either  pitchstone  or  obsidian  may  contain 
embedded  crystals  or  phenocrysts  which  can  be  recognized. 
As  in  felsite  porphyries  the  amount  may  vary  widely  from 
cases  where  they  are  rare  and  widely  scattered  to  those  in 
which  the  rock  is  thickly  strewn  with  them.  Such  por- 
phyries, consisting  of  a  glass  base  and  phenocrysts,  are 
called  vitrophyre.  Perlite  porphyries  are  known,  but  are 
rare.  The  glassy  base  of  vitrophyre  has  the  properties 
of  the  obsidian  or  pitchstone  previously  described;  it 
often  contains  spherulites  in  addition  to  the  phenocrysts. 
Of  the  latter  felclspar  is  the  most  common;  it  is  very  apt  to 
be  limpid  with  a  glassy  habit;  the  cleavage  distinguishes 
it  from  quartz,  which  may  also  occur,  sometimes  alone  and 
sometimes  with  the  feldspar.  If  phenocrysts  of  a  ferro- 
magnesian  mineral  are  present  it  is  usually  biotite,  less 
commonly  hornblende,  while  pyroxene,  though  known,  is 
rare.  The  varieties  are  usually  named  according  to  the 
prevailing  phenocryst  without  regard  to  the  character  of 
the  groundmass;  so  we  have  quartz-vitrophyre,  feldspar- 
vitrophyre,  quartz-biotite-vitrophyre,  etc.  The  phenocrysts 
are  generally  rather  small. 

The  chemical  composition  of  the  vitrophyres  is  similar 
on  the  one  hand  to  the  felsites  and  on  the  other  to  the  pure 


268 


ROCKS  AND  ROCK  MINERALS 


glasses;  they  represent  an  intermediate  stage  of  develop- 
ment, as  may  be  seen  from  the  following  table,  which  shows 
the  relations  of  all  these  varieties  of  extrusive  rocks  or 
lavas  to  one  another. 


Conditions  under  which  Magma  Cooled 
and  Solidified. 

No  Formation  of 
Crystals  in  the 
Depths  Before 
Extrusion,  no 
Phenocrysts. 

Crystals  Formed 
in  Depth  Before 
Extrusion  and 
Brought  up  by 
Magma;  Pheno- 
crysts. 

No  crystallization  of  magma  on 
surface,  on  account  of  rapid 
cooling.  —  Glassy  texture. 

Pitchstone  and 
Obsidian 

Vitrophyre. 

Crystallization  of  magma  on  sur- 
face; slower  cooling. 
—  Stony  texture. 

Felsite 

Felsite-por- 
phyry. 

Tachylite.  As  previously  stated,  basaltic  magmas  crystallize 
easily  and  rarely  form  glass,  or  only  in  relatively  small  volume. 
Basaltic  glass  is,  however,  seen  occasionally  as  a  thin  marginal  facies 
or  selvage  in  dikes,  on  lava  flows,  or  among  the  products  of  basalt- 
yielding  volcanoes,  like  those  in  Hawaii.  It  is  known  by  the  name 
of  tachylite. 

Occurrence  of  Glassy  Bocks.  The  glasses  are  found  in 
those  regions  which  are,  or  have  been  in  the  past,  scenes  of 
volcanic  activity.  While  obsidian  and  pitchstone  occur 
as  independent  flows  and  masses  near  volcanic  vents,  the 
glassy  rocks  in  general  form  only  the  upper  surface  of 
lava  sheets,  which  become  crystalline  as  they  are  penetrated 
downward;  they  are  also  found,  especially  in  pumiceous 
forms,  in  the  fragmental  material  ejected  by  volcanoes. 
To  attempt  to  name  all  the  different  occurrences  would  be 
impracticable,  but  it  may  be  mentioned  that  obsidian  in 
large  masses  is  found  in  the  Yellowstone  Park  and  is 
known  for  its  beautiful  spherulites  and  lithophysae;  at 
Mono  .Lake  in  California;  Glass  Butte,  Oregon;  White 


PLATE  23. 


A.   VOLCANIC   BOMB. 


B.  SCORIA. 


DESCRIPTION  OF  IGNEOUS  ROCKS  269 

Mountains,  Utah;  Tewan  Mountains,  New  Mexico,  and 
various  other  places  in  the  United  States;  in  Mexico, 
Iceland,  Lipari  Islands,  Italy,  Hungary,  New  Zealand, 
Transcaucasia,  etc.  Pitchstone  occurs  in  Colorado  near 
Georgetown  and  at  Silver  Cliff;  well  known  localities  are 
on  the  Island  of  Arran  off  the  west  coast  of  Scotland,  in 
Ireland,  and  at  Meissen  and  Tharandt  near  Dresden, 
Germany.  Perlites  and  pumice  are  also  found  in  the 
Yellowstone  Park;  in  Hungary,  Italy,  Iceland,  Japan,  etc. 
Basaltic  glasses  occur  on  the  west  coast  of  Scotland,  in 
Iceland  and  especially  in  the  Hawaiian  Islands. 

Alteration  of  Glassy  Bocks.  It  has  been  found  by 
microscopic  and  field  study  that  ancient  lavas,  in  a  variety 
of  places,  were  once  glassy,  though  not  so  at  present.  It 
appears  that  when  the  natural  glasses  are  exposed  to  the 
various  agencies  which  tend  to  alter  rocks,  such  as  pressure, 
heat,  action  of  water,  etc.,  they  undergo  a  slow  change, 
the  glass  is  converted  into  an  intimate  mixture  of  exces- 
sively fine  particles  of  quartz  and  feldspar,  and  loses 
entirely  its  vitreous  character.  It  then  assumes  the  stony 
texture  and  becomes  a  dense  felsite.  This  change  is 
called  devitrification.  While  the  former  glassy  condition 
of  many  felsites  cannot  be  proved,  even  microscopically, 
it  may  often  be  suspected  in  them,  from  the  presence  of 
chains  of  spherulites,  flow  structures  and  lithophysae, 
which  may  be  seen  megascopically,  and  give  strong  hints 
of  their  former  character.  Ancient  altered  lavas  of  this 
kind  have  been  described  from  the  coast  of  Maine;  from 
South  Mountain,  Pennsylvania;  from  Wisconsin;  from 
Sweden  and  other  places.  In  Sweden  they  have  been 
called  hdlleflinta,  though  this  name  is  also  used  to 
designate  somewhat  similar  rocks  of  a  different  origin. 

Fragmental  Volcanic  Rocks. 

Origin.  The  fragmental  igneous  rocks  represent  the  ma- 
terial thrown  out  by  volcanoes  during  periods  of  activity. 
The  explosive  action  is  due  to  vapors,  chiefly  that  of 


270  ROCKS  AND  ROCK  MINERALS 

water,  which  is  contained  under  pressure  in  the  magma, 
and  as  the  latter  rises  to  the  surface  and  the  pressure  is 
relieved,  departs  with  violence.  While  the  major  part  is 
passing  off  in  great  volumes,  which  rush  upward  and  carry 
the  solid  or  liquid  materials  to  great  heights,  a  minor  part 
is  also  expanding  in  the  liquid,  converting  it  into  cellular 
vesicular  forms.  Consequently  the  solid  particles  as  they 
fall  are  commonly  found  to  be  of  spongy  consistency,  but 
mixed  with  them  are  often  seen  compact  pieces  of  lava 
and  other  rocks,  parts  of  the  solid  lava  crust  formed  by 
cooling  after  a  previous  eruption,  mingled  with  fragments 
torn  from  the  rock  walls  of  the  conduit.  As  the  lava  con- 
tinues rising,  the  greater  volume  of  the  gases  may  pass  off, 
the  explosive  activity  ceases,  and  the  projection  of  material 
may  be  succeeded  by  quiet  outflows  of  liquid  rock.  Hence 
it  is  very  common  to  find  the  beds  of  fragmental  material 
interspersed  with  layers  of  compact  lava,  felsite  or  basalt. 
In  this  connection  it  should  be  also  mentioned  that  the 
chemical  composition  of  the  magma  plays  a  considerable 
part  in  explosive  activity.  Those  magmas  which  corre- 
spond to  felsite,  and  are  high  in  silica  are,  as  has  been  men- 
tioned, very  viscous  at  temperatures  where  those  low  in 
silica,  such  as  the  basalt  magmas,  which  are  rich  in  iron, 
magnesia  and  lime,  are  still  relatively  very  liquid.  From 
the  former  the  vapors,  on  account  of  their  thick  viscous 
condition,  escape  with  difficulty,  and  with  explosive 
violence;  from  the  latter  they  pass  off  readily  and  easily 
without  explosive  activity.  While  there  are  many  ex- 
ceptions to  this,  it  may  be  accepted  as  a  general  rule,  and 
we  therefore  find  that  vents  yielding  felsite  lavas  generally 
build  high  and  steep  cones,  composed  chiefly  of  frag- 
mental materials,  while  basaltic  ones  are  built  up  largely 
of  liquid  outflows  and  are  therefore  low  and  broad.  Many 
volcanoes,  like  Vesuvius,  are  of  intermediate  character  in 
which  explosion  and  projection  of  material  is  succeeded 
by  flows  of  lava,  and  the  cone  is  consequently  of  com- 
posite character. 


DESCRIPTION  OF  IGNEOUS  ROCKS  271 

Classification.  The  particles  of  magma  driven  into  the 
atmosphere  and  solidified  and  the  pieces  of  rock  are  of  all 
dimensions  from  the  finest  dust,  which  may  float  for  years, 
to  huge  masses  weighing  several  hundred  pounds.  By 
general  usage,  for  the  sake  of  convenience,  the  following 
sizes  are  roughly  distinguished:  pieces  the  size  of  an  apple 
or  larger  are  called  bombs;  those  the  size  of  a  nut  are 
termed  lapilli;  those  the  size  of  small  peas  or  shot,  ashes; 
the  finest  is  known  as  volcanic  dust.  Sometimes  the 
bombs,  lapilli,  etc.,  are  sharply  angular  and  sometimes 
smoothly  rounded  off  —  a  form  caused  by  the  grinding 
and  attrition  of  the  pieces  upon  one  another  in  the  upward 
rush  from  the  volcanic  throat.  They  should  be  dis- 
tinguished from  bombs  which  have  been  afterwards 
rounded  by  the  action  of  running  water.  The  larger 
bombs  sometimes  present  a  sub-angular  appearance,  are 
porous  and  their  surface  is  penetrated  by  cracks  as  shown 
in  Plate  23.  Such  have  been  called  bread-crust  bombs. 
The  ashes,  and  lapilli  which  usually  make  up  the  greater 
part  of  the  material  are  frequently  spoken  of  as  volcanic 
cinders  and  cones  composed  of  them  are  called  cinder  cones. 

In  a  sense,  of  course,  this  loose  material  may  be  estimated 
as  a  rock  formation;  so  far  as  the  individual  pieces  are 
concerned  they  are  to  be  considered  merely  as  fragments 
of  the  various  kinds  of  rocks  treated  in  the  foregoing 
pages,  to  be  named  and  described  as  there  set  forth. 

But  in  process  of  time  great  accumulations  of  such 
material  may  be  spread  over  wide  tracts  of  country, 
covering  up  existing  rock  formations.  The  heavier  and 
coarser  particles  fall  first,  then  the  finer,  giving  a  grada- 
tion from  top  to  bottom  and,  as  successive  outbursts  occur, 
there  is  produced  in  this  way  a  rough  bedding.  By  its 
own  weight  as  it  accumulates,  aided  by  the  action  of 
percolating  water  which  may  carry  and  deposit  substances 
in  solution,  it  gradually  becomes  compacted  into  a  more 
or  less  firm  mass,  having  a  certain  individuality  as  a  kind 
of  rock  and  deserving  of  special  treatment.  When  the 


272  ROCKS  AND  ROCK  MINERALS 

rock  is  composed  entirely  of  the  finer  particles,  dust  and 
ash,  it  is  called  volcanic  tuff;  when  this  is  mixed  with  the 
coarser  bombs  and  lapilli  it  is  termed  volcanic  conglomer- 
ate, or  better,  volcanic  breccia,  with  reference  to  the  broken 
angular  character  of  the  embedded  fragments. 

Volcanic  Tuff.  This  is  generally  a  fine-grained  rock, 
light  in  weight,  and  often  of  a  chalky  consistency,  some- 
times dense,  compact,  and  breaking  into  small  chips.  The 
color  is  usually  light,  white,  pink,  pale-brown,  gray  or 
yellow,  sometimes  passing  into  darker  shades.  The  more 
compact  varieties  may  be  easily  mistaken  for  f elsite  lavas ; 
it  is  possible,  indeed,  in  some  cases,  that  they  cannot  be 
distinguished  from  them  megascopically,  but  generally 
attentive  examination  with  a  good  lens  will  reveal  angular 
particles  of  quartz,  feldspar,  and  often  other  minerals  in 
them,  and  possibly  small  fragments  of  other  rocks.  When 
breathed  upon,  they  usually  exhale  a  strong  argillaceous 
odor,  probably  owing  to  partial  or  complete  alteration 
of  feldspathic  particles  to  clay.  When  not  too  compact 
they  have  a  rough  feel  and  yield  a  gritty  dust,  when 
strongly  rubbed  between  the  fingers,  unlike  the  smooth- 
ness of  pure  clay  or  chalk,  owing  to  the  hard,  angular 
character  of  the  dust  particles.  Sometimes  such  tuffs  con- 
tain fossil  remains  of  vegetation,  when  they  have  fallen 
upon  land  surfaces  covered  with  it,  and  carbonaceous 
remains  of  stems,  twigs,  or  leaf  imprints  may  be  found 
in  them.  If  the  material  has  fallen  into  water  the  tuff 
may  be  rich  in  various  kinds  of  fossils,  such  as  marine 
organisms,  of  possibly  great  perfection  of  form,  and  for 
the  same  reason  it  may  be  well  stratified.  All  of  these 
varied  characters,  including  the  mode  of  occurrence  and 
relation  to  other  rocks,  must  be  taken  into  account  in 
judging  the  nature  of  the  deposit. 

Volcanic  Breccia.  This  has  a  base  or  cement  of  tuff, 
more  or  less  completely  filled  with  lapilli  of  angular 
shapes,  and  these  are  often  mingled  with  larger  bombs 
and  masses  which  are  apt  to  be  rounded.  Interspersed 


PLATE  24. 


DESCRIPTION  OF  IGNEOUS  ROCKS  273 

with  these  are  apt  to  be  fragments  of  other  rocks,  pieces 
of  the  basement  through  which  the  conduit  has  drilled, 
of  limestones,  shales,  sandstones,  and  massive  crystalline 
rocks,  granite,  gneiss,  schist,  etc.  They  have  therefore  a 
strongly  conglomeratic  aspect,  like  the  specimens  seen  in 
Plate  32.  Even  when  these  rocks  have  been  greatly 
indurated  by  contact  metamorphism,  or  other  agencies, 
they  still  reveal,  by  differences  of  color  and  texture  on  a 
freshly  broken  face,  the  angular  shapes  of  the  fragments  and 
their  composite  character.  When  not  too  indurated  they 
are  apt  to  erode  very  unevenly;  the  finer  cement  being 
less  resistant  washes  away  first,  leaving  the  contained 
masses  projecting,  and  in  this  way  along  the  edges  of 
cliffs  strange  and  weirdly  shaped  figures  of  erosion  are 
produced,  called  "  hoodoos  "  in  the  Rocky  Mountains' 
region.  The  colors  of  these  breccias  is  variable,  browns, 
reds  and  chocolate  being  common,  along  with  lighter  tones, 
depending  partly  on  the  state  of  oxidation  of  the  iron- 
bearing  compounds  they  contain,  and  partly  on  the  nature 
of  the  magma,  whether  felsitic,  which  tends  to  lighter 
colors,  or  basaltic  which  produces  darker  ones. 

Occurrence  of  Tuffs  and  Breccias.  These  rocks  are  of 
wide  distribution,  being  found  in  all  regions  where  volcanic 
activity  has  taken  place;  their  presence  indeed  is  the  best 
confirmation  in  many  regions  of  such  activity  in  the  past. 
In  places  where  vulcanism  is  still  active,  or  has  only 
recently  ceased,  they  are  represented  by  the  still  uncom- 
pacted  material,  but  no  definite  line  can  be  drawn  be- 
tween the  different  conditions  of  consolidation. 

In  the  eastern  United  States,  tuffs  and  breccias  have 
been  found  in  several  localities  in  Maine;  near  Boston;  and 
at  South  Mountain,  Penn.  They  occur  also,  to  a  limited 
extent,  with  the  Triassic  eruptives  of  the  Connecticut 
Valley.  Further  research  will  probably  reveal  other 
localities,  but  they  are  neither  common  nor  conspicuous 
rocks,  being  limited  in  volume  and  so  greatly  changed  in 
character  by  various  agencies  that  in  many  places  their 


274  ROCKS  AND  ROCK  MINERALS 

true  character  is  difficultly  recognizable.  They  probably 
had  once  a  much  greater  extension,  but  erosion  has  mostly 
carried  them  away,  during  the  vast  period  of  time  which 
has  elapsed  since  volcanic  activity  was  displayed. 

In  western  America,  however,  the  case  is  very  different; 
in  the  various  ranges  of  the  Rocky  Mountains;  in  the 
Coast  and  Cascade  Ranges,  and  in  fact  over  most  of  western 
North  America,  these  are  common  rocks  and  in  many 
places  in  Colorado,  Wyoming  and  Montana,  they  occur 
in  immense  deposits,  forming  often  an  important  factor  in 
building  up  the  bulk  of  the  mountain  masses.  They  are 
especially  well  displayed  in  western  Wyoming,  in  the  region 
of  the  Yellowstone  Park,  where  the  serried  peaks  of  the 
Absaroka  Range  are  mostly  carved  out  of  tuffs  and 
breccias  aggregating  thousands  of  feet  in  depth,  thus 
testifying  to  the  enormous  volcanic  energies  which  this 
region  formerly  displayed.  A  section  cut  into  them 
by  erosion,  exhibiting  their  rough  bedding,  is  shown  in 
Plate  24.  In  this  region  they  are  frequently  interbedded 
with  flows  of  lava. 

In  Europe  tuffs  and  breccias  have  a  wide  extension. 
They  occur  in  many  places  in  the  British  Islands,  as  in  the 
Lake  district  in  northern  England,  and  associated  with 
the  volcanic  rocks  of  the  old  red  sandstone  and  Car- 
boniferous of  Scotland.  They  are  often  interbedded  with 
sedimentary  rocks  and  are  frequently  so  changed  by 
metamorphic  processes  as  to  be  recognized  only  by 
careful  petrographic  research,  having  been  changed  into 
slates,  etc.  Such  altered  tuffs  form  a  part  of  the  so-called 
"  halleflintas  "  in  Sweden  or  the  "  porphyroids  "  of 
Continental  geologists.  Tuffs  and  breccias  occur  in  many 
places  in  Germany,  France,  Italy,  etc.  The  mention  of 
these  localities  is  sufficient  to  show  their  wide  extension 
and  importance.  In  this  connection  the  reader  is  referred 
to  what  is  said  of  adobe. 


CHAPTER  VIII. 
ORIGIN  AND   CLASSIFICATION  OF  STRATIFIED  ROCKS. 

THE  stratified  rocks  consist  of  material  which  has 
already  formed  a  part  of  pre-existent  ones,  and  which  has 
been  deposited  from  some  fluid  by  which  it  has  been 
moved  from  its  former  position.  The  shifted  material 
may  have  been  moved  and  deposited  by  the  action  of 
water,  the  atmosphere,  or  glacial  ice.  The  first  case  is 
by  far  the  most  prominent  and  important,  especially  with 
respect  to  the  volume  of  the  masses  involved,  and  the 
frequency  of  their  occurrence,  and  thus  when  stratified 
rocks  are  mentioned  such  water-formed  rocks  are  always 
understood,  unless  it  is  otherwise  stated.  In  contra- 
distinction to  them,  the  material  which  has  been  moved 
and  deposited  by  the  action  of  the  atmosphere,  forms  the 
class  known  as  Aeolian  rocks,  one  of  far  less  importance. 
From  what  has  been  said,  it  is  clear  that  the  stratified 
rocks  are  secondary  ones  in  the  respect  that  their  material 
in  some  form  or  other  has  been  derived  from  already  exis- 
tent ones.  An  exception  to  this  would  be  found  in  beds 
of  coal,  which  are  truly  stratified  rocks  derived  from  plant 
life,  or  in  beds  of  volcanic  ashes  which  have  been  deposited 
from  the  atmosphere,  and  which  have  been  described  by 
preference  under  the  igneous  rocks.  But  in  general  the 
statement,  that  the  material  of  the  stratified  rocks  is  sec- 
ondary, holds  true,  and  it  has  been  derived  from  former 
rocks  of  all  classes  —  igneous,  metamorphic  and  strati- 
fied —  and  in  the  case  of  the  earliest  sediments  from  the 
earth's  original  crust,  if  such  ever  existed. 

The  rocks  which  have  been  formed  in  water  may  be 
divided  into  two  main  groups,  according  to  the  manner 

275 


276  ROCKS  AND   ROCK  MINERALS 

in  which  the  material  has  been  deposited;  they  consist 
either  of  substances  mechanically  held  in  suspension,  and 
then  directly  dropped,  or  of  that  which  has  been  in  solu- 
tion, and  through  chemical  agencies,  either  of  organic  life 
or  otherwise,  has  been  rendered  insoluble,  and  has  been 
therefore  deposited.  The  first  we  may  call  mechanical, 
the  second  chemical  sediments.  Yet  even  between  these, 
as  we  shall  see  later,  it  is  difficult  to  draw  a  definite  line. 
We  have  then  the  following  classes  to  deal  with : 

Sedimentary  rocks ;  water-formed  (  mechanical. 

i  chemical. 
Aeolian  rocks;  wind  formed       .     .    mechanical. 

Decay  of  Rocks ;  Formation  of  Soil.  When  firm  and  even 
dense  rocks  are  exposed  to  the  action  of  the  atmosphere, 
they  gradually  decay  and  are  turned  into  soil.  This  is 
brought  about  by  a  variety  of  agencies.  All  rock  masses 
are  penetrated  in  various  directions  by  cracks  and  fissures 
called  joints;  these  are  both  great  and  small,  and  in  addi- 
tion the  individual  mineral  grains  contain  cleavage  and 
other  cracks.  Thus  water  is  able  to  thoroughly  permeate 
the  rock  masses,  and  in  cold  regions  where  alternate 
thawing  and  freezing  goes  on,  the  expansion  of  the  water 
in  turning  to  ice  keeps  on  splitting  and  crumbling  the 
rocks  until  on  the  surface  they  are  reduced  to  a  mass  of 
debris.  The  expansion  and  contraction  of  rocks  in  hot 
countries  and  in  arid  regions,  under  great  daily  and  yearly 
changes  of  temperature,  accomplishes  the  same  thing  more 
slowly.  The  expansion  of  the  growing  roots  of  trees  and 
plants  tends  to  the  same  end.  By  such  processes  there  is  a 
constant  tendency  for  the  rock  masses  to  be  broken  up, 
mechanically,  into  smaller  and  smaller  fragments.  In  the 
meantime  the  substances  dissolved  in  the  water,  such  as 
air,  acids  from  decaying  vegetation,  and  especially  carbonic 
acid  gas,  are  acting  chemically  upon  the  rock  minerals,  con- 
verting the  silicates,  oxides,  and  sulphides  into  other 
forms,  into  carbonates,  hydrated  silicates,  hydroxides, 


PLATE  26. 


^•^^•E^^K  I^^^Vi^M 

&*< 

i 

«^^ 


CLASSIFICATION  OF  STRATIFIED   ROCKS         277 

sulphates,  etc.  Much  material  goes  into  solution,  is  leached 
out,  and  by  running  water  is  carried  into  lakes  and  the 
ocean,  where  it  concentrates,  and  where  we  must  again 
consider  it  under  the  formation  of  the  chemically  precipi- 
tated sediments.  Some  minerals,  such  as  quartz,  are  not 
attacked  to  any  appreciable  extent,  or  but  very  slowly, 
under  ordinary  circumstances,  and  these  remain  to  form 
the  chief  part  of  the  rock  debris.  It  is  for  this  reason 
that  silicates,  and  especially  quartz,  play  the  chief  min- 
eral r61e  in  the  sedimentary  rocks  formed  by  mechani- 
cal processes.  This  debris  of  broken,  crumbled  and 
altered  rock,  which  constitutes  a  detritus,  has  been 
called  by  various  names,  and  the  finer  upper  portion 
in  which  vegetation  grows  is  the  soil.  Under  this  latter 
name  for  convenience  we  may  consider  all  of  it.  The 
gradual  change  from  rock  below  to  soil  above  is  illus- 
trated in  Plate  17. 

Movement  of  Soil.  The  surface  of  the  land  in  general 
is  covered  by  a  mantle  of  soil  resting  on  the  rocky  crust  of 
the  earth.  The  latter,  which  is  popularly  known  as  the 
"  country  rock,"  here  and  there  in  ledges,  precipices,  and 
the  craggy  tops  of  hills  and  mountains  projects  through 
this  covering.  By  the  action  of  running  water,  aided  by 
gravity,  this  crumbled  rock  and  soil  mantle,  which  is  appar- 
ently at  rest,  is,  geologically  considered,  actually  in  motion, 
and  is  continually  being  urged  downward  into  the  sea,  its 
ultimate  goal,  Plate  25.  On  steep  slopes  it  goes  more 
rapidly,  in  valleys  more  slowly;  in  level  plains,  like  water 
in  a  lake,  it  is  temporarily  impounded.  Its  rate  of  motion 
varies  continually  from  time  to  time  and  from  place  to 
place.  Its  movement  in  mass  is  of  course  very  slow; 
when  suspended  in  running  water,  that  of  the  water  which 
carries  it;  when  resting  on  the  stream  bottom  it  varies 
according  to  circumstances.  Thus  the  land  waste  is  being 
ever  carried  away  and  ever  renewed  by  the  destruction  of 
the  rocks.  The  greater  part  is  carried  into  the  sea,  but  a 
considerable  part  is  deposited  in  inland  lakes  and  seas, 


278 


ROCKS  AND  ROCK  MINERALS 


and  on  the  lower  plains  and  deltas  of  great  rivers,  which 
from  time  to  time  are  heavily  flooded.  It  is  this  material 
which  forms  the  sedimentary  rocks  of  mechanical 
deposition. 

Gradation  of  Material.  The  detritus  of  the  land  con- 
sists of  material  of  very  variable  sizes,  and  in  northern 
countries  over  which,  glaciers  have  passed  this  is  particu- 
larly apt  to  be  the  case,  as  rock  masses  showing  great 
extremes  in  dimensions  are  moved  and  mingled  by  them. 
When  such  material  is  moved  by  running  water  it  becomes 
sorted  and  graded,  according  to  the  strength  of  the  cur- 
rent, into  masses  consisting  approximately  of  equal  sized 
particles.  When  they  are  larger  than  peas  the  material 
is  called  gravel  and  the  individual  pieces  are  termed 
pebbles;  large,  loose  pieces  of  rock  from  the  size  of  a  small 
melon  up  are  spoken  of  as  boulders.  Pieces  smaller  than 
peas,  which  form  a  non-cohering  mass  when  wet  with 
water,  are  termed  sand,  while  the  finest  particles  which 
are  readily  lifted  and  transported  by  movements  of  the 
atmosphere  are  known  as  dust,  and  these  when  wet  and 
then  dried  generally  cohere  into  solid  material.  All  these 
grade  into  one  another.  The  following  table  shows  a 
more  accurate  division  according  to  size. 


Name  of  Material. 

Diameter  in  Millimeters 

Fine  Gravel    . 
Coarse  Sand  . 
Medium  Sand 
Fine  Sand  .    . 
Very  fine  Sand 
Silt  

2-1 
1-0.5 
0.5-0.25 
0.25-0.1 
0.1-0.05 
0.05-0  01 

Fine  Silt  

0.01-0  005 

Clay     

0.005-0.0001 

Thus,  roughly  speaking,  the  material  may  be  classified 
into,  (a)  gravel,  (6)  sand,  (c)  mud,  clay  or  silt.  Since 
this  division  is  made  the  basis  of  classification  of  the 


CLASSIFICATION  OF  STRATIFIED  ROCKS         279 

mechanically  formed  sedimentary  rocks,  each  of  them  may 
be  examined  somewhat  more  in  detail. 

Gravel.  The  pebbles  which  compose  a  gravel  are 
pieces  of  individual  rocks  and  like  them  are  generally 
made  up  of  grains  of  different  kinds  of  minerals.  In 
some  cases  they  are  composed  of  only  one  mineral,  and  of 
these,  quartz  is  by  far  the  most  common.  Such  quartz 
pebbles  may  be  fragments  derived  from  quartzite  strata, 
from  a  quartz  vein,  or  from  large  quartz  crystals  from 
some  granite-pegmatite  dike.  Such  coarse  granites  or 
pegmatites  may  furnish  pebbles  consisting  of  other  single 
minerals,  especially  feldspar. 

The  form  and  appearance  of  pebbles  depends  on  the  conditions 
to  which  they  have  been  exposed.  Those  which  have  suffered 
considerable  transport  in  the  bed  of  streams,  or  have  been  rolled 
on  the  shores  of  lakes  and  of  the  sea,  are,  as  is  well  known,  rounded 
and  become  ovoid  to  spherical.  They  are  apt  to  have  a  very  smooth 
surface  with  a  characteristic  faintly  dimpled,  slightly  dented,  or 
inverted  shagreen  appearance,  caused  by  their  repeated  collisions 
under  movement.  This  is  best  seen  on  a  pebble  of  a  hard  homo- 
geneous substance,  as  in  one  of  quartz.  If  composite  in  nature 
they  are  often  pitted  by  the  decay  and  removal  of  softer  or  more 
easily  altered  particles. 

The  degree  of  rounding  shown  by  pebbles  depends  on  the  distance 
and  length  of  time  they  have  been  transported  and  on  the  hardness  of 
the  material.  Sedimentary  rocks,  as  will  be  shown,  are  sometimes 
composed  of  pebble-sized  fragments,  which  have  suffered  very  little 
movement,  and  which  still  retain  their  original  rough,  angular 
character. 

Pebbles  and  boulders  which  have  been  transported  by  glaciers 
are  sometimes  seen  in  sedimentary  rocks.  These  have  character- 
istic sub-angular  forms,  with  faces  ground  upon  them,  which  are 
polished  and  scratched  by  parallel  and  crossing  grooves  or 
scratches.  Pebbles,  partly  buried  in  the  sand  of  the  seashore 
and  of  deserts,  are  also  often  subangular  and  facetted,  the  faces 
being  ground  by  the  sand  drifting  past  them,  but  these  lack  the 
scratches. 

Pebbles  buried  in  the  soil  often  show  fern  or  moss-like  markings 
or  dendrites  upon  them,  or  are  sometimes  covered  with  a  shiny  skin 
of  dark  color.  This  comes  from  a  deposit  from  water,  of  manganese 
or  iron  oxides. 


280  ROCKS  AND  ROCK  MINERALS 

Sand.  Strictly  speaking,  sand  means  particles  of  a 
certain  size,  as  mentioned  above,  and  has  no  reference  to 
their  composition:  thus  we  have  quartz  sand,  coral  sand, 
volcanic  sand,  etc.  It  happens,  however,  that  by  far 
the  greater  part  of  the  sands  are  composed  of  particles 
of  quartz,  and  some  are  exclusively  made  up  of  it.  For 
this  reason  when  sand  is  spoken  of  briefly,  quartz  sand  is 
always  understood. 

The  composition  of  ordinary  sand  is  quite  variable,  depending 
on  the  locality.  In  addition  to  the  quartz  grains,  those  of  many 
other  minerals  are  present,  depending  on  the  rocks  of  the  region. 
Feldspar,  garnet  and  iron  ore  are  very  common.  Various  silicates 
such  as  hornblende,  pyroxene,  tourmaline,  etc.,  are  apt  to  occur. 
Some  grains  may  be  made  of  pieces  of  very  fine-grained  rocks  of 
composite  character.  Twenty-three  different  kinds  of  minerals 
were  found  in  the  dune  sands  of  Holland  by  Retgers. 

Like  pebbles  the  sand  grains  are  more  or  less  rounded,  depending 
on  the  amount  of  transport.  In  some  rather  coarse  sea  sands  they 
are  almost  all  spherical.  Below  a  certain  degree  of  fineness  the 
grains  do  not  become  more  rounded  by  attrition  in  water  among 
themselves;  this  is  due  to  the  fact  that  the  capillary  film  of  water 
covering  them  acts  as  a  buffer  and  prevents  them  from  coming  in 
contact  when  they  collide;  in  the  larger  grains  it  is  not  able  to  do 
this. 

Mud,  Silt  and  Clay.  This  consists  of  the  finest  ma- 
terial of  the  land  waste.  As -sedimentary  deposits  they 
are  characteristically  found  off  shore,  or  in  sheltered  bays 
and  sounds,  where  the  slow  movement  of  the  water  does 
not  permit  the  transport  of  the  heavier  sand  and  gravel, 
and  as  the  material  forming  the  lower  flood  plains  and 
deltas  of  great  rivers.  On  account  of  their  minute  size 
the  particles  are  little  apt  to  be  rounded,  but  under  the 
microscope  show  angular  forms.  Like  the  sands  they 
may  be  composed  of  a  great  variety  of  minerals,  kaolin, 
mica,  quartz,  feldspar,  etc.,  but  just  as  quartz  is  the  char- 
acteristic mineral  of  the  sands,  so  is  kaolin  that  of  muds 
and  clays.  As  shown  elsewhere  the  decay  of  the  feldspars 
of  the  rocks  produces  kaolin  or  clay,  while  the  quartz 


CLASSIFICATION  OF  STRATIFIED   ROCKS         281 

grains  are  unaltered;  the  clay  particles  are  excessively 
fine  and  light,  while  the  quartz  ones  are  mostly  larger  and 
heavier.  From  this  there  tends  to  be  a  separation  of  the 
two  by  moving  water;  as  the  current  slackens  the  quartz 
is  deposited  first,  forming  sand,  while  the  lighter  clays  are 
carried  beyond  and  settle  in  still  water.  Fine  flakes  of 
white  mica  are  apt  to  accompany  them. 

In  fresh  water  a  portion  of  most  clays,  consisting  of  the  very 
finest  and  lightest  particles,  will  remain  in  suspension  almost  indefi- 
nitely. Turbid  water  of  this  kind  acts  much  as  if  it  were  a  solu- 
tion of  clay  in  water ;  if  salts  be  added  to  it,  or  if  it  be  mixed  with  sea 
water,  the  clay  then  curdles  into  lumps  or  flocculates  and  is  quickly 
deposited,  leaving  the  liquid  clear.  This  behavior  is  analogous  to 
that  of  salts  in  solution,  and  it  has  an  important  bearing  on  the 
deposition  of  material  carried  into  the  sea,  and  on  the  formation  of 
certain  kinds  of  rocks. 

Muds  or  clays  are  characterized  according  to  the  pre- 
dominance of  certain  constituents;  thus  some  are  cal- 
careous, containing  more  or  less  carbonate  of  lime  and 
are  often  called  marls;  some  contain  a  good  deal  of  fine 
quartz  and  are  spoken  of  as  siliceous,  others  are  rich  in 
deposited  iron  oxides  and  are  ferrugineous  clays  or 
ochers,  while  in  many  these  constituents  are  present  in 
minimum  amount,  or  are  wanting,  and  these  are  plain 
clays  or  argillaceous  deposits.  Such  mixed  forms  are 
transitional  to  the  chemically  deposited  rocks  described 
later. 

Dissolved  Material.  The  waste  of  the  land  includes  not 
only  the  material  mechanically  transported  by  water,  but 
also  that  which  is  taken  into  solution  and  ultimately 
carried  into  the  sea.  A  rough  estimate  of  this  for  the 
continents  places  it  at  5,000,000,000  tons  per  annum. 
It  is  an  important  fraction  of  the  whole  amount  removed, 
compared  with  the  mechanical  sediments.  It  varies 
greatly  in  different  rivers,  depending  on  the  composition 
of  the  rocks  forming  their  basins.  It  is  inferred  that 
through  the  concentration  of  this  material  in  solution 


282  ROCKS  AND  ROCK  MINERALS 

during  past  ages  the  salts  now  in  the  ocean  have  been 
produced.  From  these  salts,  those  sedimentary  rocks, 
whose  material  through  chemical  agencies,  either  of 
organic  life  or  otherwise,  has  been  redeposited  from  solu- 
tion, have  been  formed.  It  includes  the  important  class 
of  carbonates,  limestones,  dolomites,  etc.,  and  the  less 
important  sulphates  and  chlorides,  such  as  gypsum  and 
rock-salt. 

It  is  probable  that  the  carbonates  of  lime,  magnesia,  and 
alkalies  were  all  originally  derived  from  silicates  of  these 
oxides.  Water  containing  carbon  dioxide  has  converted 
them  into  carbonates,  as  illustrated  under  the  decom- 
position of  feldspar,  and  has  then  dissolved  and  carried 
them  into  the  sea.  The  sulphates  have  been  formed  by 
the  oxidation  of  sulphides  in  the  original  rocks  and  the 
union  of  the  sulphuric  acid  with  the  stronger,  more 
soluble,  alkaline  bases.  The  chlorides  have  in  part  been 
derived  from  minerals  of  the  original  rocks  and,  perhaps, 
made  in  part  by  volcanic  emissions  from  deeply  seated 
magmas  within  the  earth. 

Structure  of  the  Sedimentary  Bocks.  The  sedimentary 
rocks,  as  geological  masses,  differ  greatly  from  the  igneous 
ones  in  that  they  form  widely  extended,  relatively  thin 
bodies,  making  part  of  a  coating  or  mantle  upon  the  earth's 
outer  surface;  they  never  prolong  themselves  by  extension 
into  the  depths,  as  the  latter  always  do.  It  is  thus  their 
horizontal,  as  contrasted  with  their  vertical  extension, 
which  gives  them  importance  as  geological  masses.  The 
most  characteristic  feature  about  their  structure  is  that 
they  are  stratified.  This  means  that  they  consist  of  layers, 
varying  in  material,  texture  and  color,  and  in  thickness, 
which,  if  undisturbed  by  geological  events  more  recent 
than  their  formation,  are  in  general  horizontally  disposed 
one  upon  another.  This  is  illustrated  on  Plate  26. 
It  is  due  to  the  fact  that  the  mechanical  sediments  have 
been  deposited  by  moving  currents  of  water  in  lakes  and 
seas  and  on  the  flood  plains  of  rivers,  and  these  currents, 


PLATE  26. 


CLASSIFICATION  OF  STRATIFIED  ROCKS         283 

acting  with  diverse  material  and  continually  varying  in 
strength,  have  assorted  the  sediments  and  deposited  them 
in  strata.  For  further  consideration  of  this  subject 
information  should  be  sought  in  any  of  the  numerous 
manuals  of  Geology,  but  it  may  be  stated  as  a  general 
law  that  sedimentary  deposits  are  always  stratified  and 
that,  conversely,  perfect  stratification,  resulting  from  an 
assortment  of  particles,  is  regarded  as  a  proof  of  deposit 
in  water. 

Volcanic  ash  deposits  are  commonly  rudely  and  sometimes  well 
stratified;  the  heavier  of  the  particles  projected  upward  falling  first 
to  be  succeeded  by  smaller  lighter  ones;  repetitions  of  the  process 
make  individualized  beds  and  thus  a  rude  stratification  (see  Plate 
24 ,)  while  the  lighter  dust  may  be  deposited  by  moving  air  currents 
much  as  if  in  water. 

For  stratification  variation,  both  of  conditions  and  in  the  size  of 
the  particles,  is  necessary;  during  a  period  in  which  uniformity  pre- 
vails in  either  it  will  be  wanting.  Thus  aeolian  deposits,  which 
consist  of  the  finest  sands  and  dust  driven  by  the  wind,  are  often  so 
uniform  with  respect  to  the  size  of  the  particles  in  any  one  area  that 
no  stratification,  or  but  very  little,  is  produced.  In  the  same  way 
deposits  of  carbonate  of  lime,  as  limestone  and  chalk,  in  the  open 
ocean  may  take  place  under  such  uniform  conditions  and  size  of 
particles  that  beds  of  these  rocks,  perhaps  a  hundred  feet  in  thick- 
ness or  more,  are  quite  structureless  and  devoid  of  apparent  strati- 
fication throughout  that  extent. 

Mere  parallelism  of  layers  in  a  rock  is  not  in  itself  a  mark  of 
stratification  and  therefore  a  proof  that  the  rock  exhibiting  it  is  of 
sedimentary  origin.  Its  mineral  composition,  texture,  and  relation 
to  the  accompanying  rock  masses,  and  to  the  general  geology  of  the 
region,  must  also  be  taken  into  consideration.  For  the  spreading 
action  of  flowing  lava  may  draw  out  portions  of '  unlike  character 
within  it  into  thin  superimposed  sheets  as  illustrated  in  Plate  22. 
Such  lavas  when  fresh  are  easily  recognizable,  but  buried  in  the 
midst  of  sedimentary  deposits  and  changed  in  appearance  by  geologic 
ages  of  exposure  to  various  agencies,  they  may  be  confused  with  the 
accompanying  stratified  rocks.  And  again  very  perfect  parallelism 
of  layers  and  structure  may  be  induced  in  all  kinds  of  rocks  by  the 
shearing  and  metamorphism  accompanying  movements  of  the 
earth's  crust  and  mountain  building.  These  may  superficially 
simulate  stratification  quite  perfectly,  but  consideration  of  the  points 
mentioned  above  is  generally  sufficient  to  show  the  difference 


284  ROCKS  AND  ROCK  MINERALS 

between  them.  Many  serious  errors  in  understanding  the  real 
origin  of  the  rocks  of  different  places  and  their  geology  have  occurred 
in  the  past  through  failure  to  properly  appreciate  these  facts. 

The  individual  layers  of  stratified  rocks,  which  are  uni- 
form in  texture,  color  and  composition,  may  vary  from  the 
thinness  of  paper  to  a  hundred  feet  or  more.  Usually, 
it  will  be  observed  that  a  certain  layer,  which  has  a  general 
similarity  of  character  and  composition  that  serve  to 
distinguish  it  clearly  from  others  above  and  below,  is 
made  up  of  much  smaller  subdivisions  whose  differences 
from  one  another  are  not  very  marked.  The  larger  division 
is  usually  known  as  a  layer  or  bed,  the  smaller  ones  are 
termed  lamince.  The  main  differences  between  laminae 
are  generally  in  coloration;  as  shown  in  Plate  33,  between 
beds  in  texture  and  composition.  As  explained  above, 
under  uniform  conditions  lamince  may  be  wanting  in  a 
particular  bed.  The  general  homogeneity  of  a  bed  is 
shown  by  its  particular  hardness  and  appearance,  its 
individual  method  of  cracking  or  jointing,  and  the  way 
in  which  it  is  affected  by  erosion,  which  differs  from  the 
beds  above  and  below  it.  A  collection  of  beds  lying  con- 
cordantly  one  above  another,  and  deposited  during  a  given 
geological  period  of  time,  is  called  a  formation. 

Texture  of  Sedimentary  Rocks.  This  depends  upon  the 
relative  size  of  the  particles,  which  determine  the  fineness 
or  coarseness  of  grain,  upon  their  shape,  and  upon  the 
amount  and  character  of  the  cement,  which  determines  the 
firmness  or  friability  of  the  rock.  The  size  of  grain  varies 
within  wide  bounds,  but  as  explained  previously  under 
gradation  of  material,  this  in  itself  determines  largely  the 
kind  of  rock.  Thus  conglomerates  are  of  necessity 
coarse-grained  rocks;  sandstones,  medium-grained  ones, 
and  shales,  fine-grained  or  compact.  Still  within  the 
limits  of  each  class  there  is  variation  in  this  respect  and 
we  are  accustomed  to  speak  of  fine,  medium  and  coarse- 
grained sandstones;  a  medium  grain  in  this  rock  is  about 
that  of  ordinary  loaf  sugar. 


CLASSIFICATION  OF  STRATIFIED  ROCKS         285 

The  shape  of  the  component  grains,  when  these  are 
megascopically  visible,  depends  on  the  amount  of  trans- 
port which  they  have  suffered,  as  explained  under  gravel. 
Usually  they  are  more  or  less  rounded  or  ovoid,  but 
sometimes  quite  angular.  The  latter  is  more  apt  to  be 
the  case  as  the  size  of  grain  increases.  Sometimes  this 
broken,  angular  character  of  the  particles  can  be  dis- 
tinctly seen  in  medium-grained  sandstones  and  arkoses 
by  close  observation  with  a  good  lens.  It  shows  the  rock 
to  have  a  distinctly  clastic  nature.  In  the  case  of  coarser 
rocks  and  in  conglomerates  it  becomes  very  striking,  and 
such  rocks  are  called  breccias  and  are  said  to  have  a 
brecciated  structure.  This  is  illustrated  in  Plate  32. 
Such  breccias  are  not  to  be  confused,  however,  with 
volcanic  breccias,  as  described  on  page  272. 

The  cement  is  that  which  binds  the  particles  of  sedi- 
mentary rocks  together  and  converts  them  from  loose 
material  into  firm  rock.  Various  substances  act  in  this 
capacity,  according  to  circumstances;  sometimes  it .  is 
carried  into  the  rock  from  outside  sources  in  solution 
and  deposited  in  its  pores,  sometimes  part  of  the  sedi- 
ment itself  goes  into  solution  and  is  redeposited,  and  some- 
times it  consists  of  fine  material  mechanically  enclosed 
with  the  sediment.  In  the  first  and  second  cases  silica 
and  carbonate  of  lime  are  common  binding  materials,  in 
the  third,  clay  or  clay-like  substances  perform  this  func- 
tion. Iron  oxide,  probably  according  to  the  second  case, 
is  also  a  not  uncommon  cement  in  the  form  of  hematite, 
or  gothite,  or  limonite.  The  fine  deposits  of  mud  and 
clay  appear  to  be  able  to  consolidate  into  firm  rocks, 
under  the  pressure  of  superincumbent  masses,  without 
the  presence  of  a  perceptible  cement,  though  it  is  some- 
times present. 

The  firmness  of  the  rock  depends  then,  in  part  on  the 
amount  of  cement  and  its  quality,  and  in  part  on  the  pres- 
sure. As  a  result  all  degrees  of  this  character  are  shown 
by  sedimentary  rocks;  some  are  very  hard,  firm  and 


286  ROCKS  AND  ROCK  MINERALS 

compact,  breaking  like  igneous  rocks  under  the  hammer 
and  susceptible  of  a  polish,  as  in  the  case  of  some 
limestones  and  sandstones,  while  others  are  so  loose, 
incoherent  and  friable  that  they  may  be  readily  rubbed  to 
powder  under  the  fingers,  as  with  chalks  and  some  sand- 
stones. And  all  gradations  may  be  found  between  these 
extremes. 

Color  of  Sedimentary  Rocks.  This  depends  partly  on 
the  color  of  the  constituent  mineral  grains  or  particles,  and 
partly  on  included  substances  which  act  as  a  pigment. 
The  most  common  minerals  which  form  the  sedimentary 
rocks  are  quartz,  kaolin,  feldspar,  calcite  and  dolomite; 
these  are  white  or  colorless  substances  naturally,  though 
they  sometimes  display  exotic  coloration,  and  rocks  com- 
posed purely  of  them,  without  included  pigment,  are  white, 
as  illustrated  by  certain  sandstones,  clays  and  chalk. 
Generally  more  or  less  pigment  is  present,  and  the  common 
ones  are  the  oxides  of  iron  and  carbonaceous  matter. 
The  iron  occurs  in  the  form  of  ferric  oxides,  or  hydrated 
oxides,  as  hematite,  or  probably  hydrohematite  (turgite), 
which  gives  red  to  red-brown  colors,  or  as  limonite,  or 
gothite,  which  produce  yellow  to  yellowish  brown  tones. 
Carbonaceous  matter  or  finely  divided  carbon  is  black, 
and  this  is  the  color  of  the  rock,  if  it  contains  an  excess  of 
it;  as  the  amount  lessens  dark  grays  are  formed,  and  so 
on  into  pale  grays.  If  both  organic  carbonaceous  matter 
and  iron  oxides  are  present  in  the  rock,  the  former  exerts 
a  controlling  power  over  the  coloring  capacity  of  the  latter 
in  this  way;  in  the  presence  of  organic  matter,  especially 
when  it  is  decaying,  iron  is  reduced  from  the  ferric  to  the 
ferrous  condition,  it  changes  from  ferric  oxide  to  ferrous 
carbonate,  and  as  ferrous  compounds  are  colorless  or  light- 
colored  the  rock  has  the  tones  of  color  produced  by  the 
carbonaceous  pigment.  If  such  rocks  are  exposed  to 
weathering  and  the  carbonaceous  material  destroyed,  the 
iron  is  reoxidized  and  the  red  and  yellow  colors  show 
themselves.  This  is  illustrated  in  the  outcrops  and  on  the 


CLASSIFICATION  OF  STRATIFIED  ROCKS         287 

joint  faces  of  many  black  slates  which  weather  red  or 
yellow.  On  the  other  hand  if  the  rocks  are  devoid  of  iron, 
when  the  organic  pigment  bleaches  out,  they  become  white 
or  very  light  in  color.  And  again,  if  solutions  containing 
organic  matter  leach  through  the  rocks,  the  iron  oxide 
is  not  only  changed  into  the  ferrous  condition,  but  when 
reduced  to  this  state,  or  in  it  originally,  goes  into  solution 
also  and  is  carried  out,  the  rocks  thus  becoming  light  or 
colorless. 

The  most  common  colors  then  for  the  sedimentary 
rocks  are  white  to  light  gray,  to  dark  gray  and  black,  or 
from  white  to  pink  into  red,  to  dark  red  and  red-brown,  or 
from  pale  yellow  to  buff,  to  yellow-brown.  The  reds  and 
yellows  are  often  seen  commingled  in  the  same  rock  mass 
or  layer,  according  to  the  varying  iron  hydroxides.  In 
the  case  of  conglomerates  and  coarse  arkose  sandstones, 
these  colors  may  be  modified  by  those  of  the  frag- 
ments of  the  unchanged  original  rocks  which  they  may 
contain. 

Chemically  formed  Rocks.  These  rocks  are  formed  in 
those  cases  where  material,  which  has  been  in  solution,  has 
become  insoluble  by  reason  of  some  agency,  and  is  pre- 
cipitated. The  chief  agencies  involved  are  concentration 
of  the  solutions  and  organic  life.  In  the  latter  case  ani- 
mals living  in  water,  chiefly  in  the  sea,  secrete  inorganic 
material  in  the  production  of  their  hard  parts,  either  skele- 
tons to  stiffen  them,  or  shells  as  defensive  armor  for  their 
soft  organisms.  As  the  animals  die  these  collect  as  depos- 
its. The  chief  substances  secreted  are  carbonate  of  lime, 
CaCO3,  and  silica,  Si02,  the  former  being  much  the  more 
abundant  and  important.  Examples  are  seen  in  the  for- 
mation of  reefs  and  islands  by  corals,  and  in  the  shell- 
banks  made  by  mollusks.  Vegetable  organisms  also, 
under  certain  conditions,  secrete  silica,  and  give  rise  to 
deposits  of  that  substance. 

The  deposits  produced  by  concentration  occur  when 
bodies  of  sea-water  are  isolated  from  the  ocean  by  geol- 


288  ROCKS  AND  ROCK  MINERALS 

ogic  processes  and  become  so  concentrated  by  evapora- 
tion that  they  are  no  longer  able  to  retain  the  salts 
in  solution.  These  are  then  deposited  in  the  order  of 
their  solubility.  Gypsum  and  anhydrite,  sulphates  of 
lime,  and  common  salt,  sodium  chloride,  are  the  most 
important  substances  deposited  in  this  way.  The  same 
result  occurs  in  lakes  and  inland  seas  in  arid  regions, 
which  have  no  outlet  and  where  there  is  a  steady  concen- 
tration of  material  in  solution,  brought  into  them  by  in- 
flowing streams.  Carbonates,  sulphates  and  chlorides 
are  the  main  salts  deposited.  In  a  somewhat  similar 
manner,  when  water  passing  through  the  rocks  becomes 
mineralized  by  taking  substances  into  solution  and  then 
attains  the  outer  air,  as  in  springs,  these  substances  are 
deposited.  Such  deposits  are,  with  respect  to  the  masses 
involved,  geologically  speaking,  of  minor  importance,  and 
are  illustrated  by  the  deposits  of  carbonate  of  lime  around 
springs,  and  in  caves,  and  of  silica  around  geysers  and  hot- 
springs  in  volcanic  regions.  A  more  important  case  is 
where  water,  in  the  presence  of  organic  matter,  has  leached 
iron  oxide  from  the  rocks  and  soils  and  carrying  it  in 
solution  into  swamps  and  shallow  waters,  has  there  depos- 
ited it,  either  in  the  form  of  ferrous  carbonate  (siderite), 
if  there  is  excess  of  organic  matter  present,  or  in  the  reoxi- 
dized  form  of  ferric  hydroxide  (limonite)  if  it  is  wanting. 
By  this  means  widely  extended  beds  of  iron  ore  have 
been  formed,  which  are  of  great  technical  value  and 
importance. 

Circulation  of  Material.  Geological  science  is  not  yet 
in  a  position  to  state  definitely  concerning  the  origin  of 
the  material  of  the  earliest  formed  sediments  upon  the 
earth.  We  have  only  the  fact  that,  wherever  upon  the 
continents  the  deepest  amounts  of  erosion  have  occurred 
and  the  basement  upon  which  the  visibly  earliest  sedi- 
ments have  been  deposited  is  exposed,  this  basement  is  of 
igneous  rock  or  of  apparently  igneous  rock  which  has  been 
metamorphosed,  and  the  sediments  such  as  could  have 


CLASSIFICATION  OF  STRATIFIED  ROCKS        289 

been  derived  from  its  erosion  and  weathering.  What- 
ever the  nature  of  the  original  sediments  was,  it  is  evident 
that  when  they  had  been  elevated  to  form  land,  since 
erosive  processes  continued,  any  new  sediments  would  be 
derived  from  the  old  ones  plus  any  material  that  would  be 
added  by  the  continued  erosion  of  such  areas  of  the 
original  surface  as  the  first  sediments  had  not  covered  and 
which  still  remained  land,  and  of  any  fresh  igneous  rocks 
which  had  come  up  to  occupy  a  place  in  either.  This  con- 
dition of  affairs  has  continued  to  the  present  time;  sedi- 
ments have  been  laid  down,  and  then  elevated  to  form 
land,  sometimes  being  greatly  metamorphosed  in  the 
process  and  sometimes  not,  and  these  by  their  erosion 
have  in  turn  yielded  fresh  sediments,  and  so  on.  Thus 
there  has  been  a  circulatory  round  of  material,  with 
changes  of  conditions  to  affect  the  minerals  at  each  stage, 
and  only  the  most  resistant,  such  as  quartz,  have  been 
able  to  undergo  it  without  change.  One  is  a  downward 
course  from  land  to  sea;  the  return  journey  is  the  ascen- 
sion of  the  land  from  the  sea.  The  silicate  minerals,  which 
chiefly  form  the  mechanical  sediments,  have  performed  the 
downward  journey  in  suspension,  the  carbonate  minerals, 
on  the  other  hand,  have  made  it  mainly  in  solution.  This 
means  that  sandstone,  for  example,  on  erosion  is  mostly 
carried  away  mechanically,  while  limestone,  which  consists 
mainly  or  entirely  of  carbonate  of  lime,  ultimately  disap- 
pears mostly  by  going  into  solution,  although  at  the  begin- 
ning of  erosive  work  upon  it,  it  may  be  largely  mechanical 
processes,  which  break  down  the  rock.  Some  cases  of 
mechanical  sediments  consisting  of  carbonate  of  lime  occur, 
though  not  relatively  of  great  importance,  and  these  are 
described  under  limestone,  along  with  some  deposits  of 
lime  formed  on  land  by  evaporation,  which  may  be 
regarded  as  temporary  stoppages  of  the  material  in  solu- 
tion on  its  way  to  the  sea.  This  latter  case  is  illus- 
trated in  the  formation  of  travertine  around  springs  and 
in  caves. 


290       ROCKS  AND  ROCK  MINERALS 

Minerals  of  the  Sedimentary  Rocks.  From  what  has 
been  said  in  the  foregoing  pages,  it  is  evident  that  the 
minerals  of  the  sedimentary  rocks  consist  of  those  which 
compose  the  igneous  ones  and  which  have  been  able  to 
endure  without  change  the  various  conditions  to  which 
they  have  been  subjected,  as  well  as  the.  new  ones  formed 
by  weathering  and  erosion.  The  finer  the  material  and 
the  longer  the  time  of  its  transport  has  been,  the  more 
thoroughly  it  will  be  changed  into  new  mineral  combina- 
tions. Hence  quartz  and  feldspar  are  important  in  the 
coarser-grained  rocks,  quartz,  kaolin  and  mica  in  the  finer- 
grained  ones;  while  calcite,  dolomite,  siderite,  limonite 
and  gypsum  represent  minerals  of  the  chemical  deposits. 
In  the  fine-grained  and  dense  sedimentary  rocks,  formed 
of  silts,  muds  and  clays,  the  particles  are  so  fine,  that  from 
the  megascopic  point  of  view  the  mineralogical  composi- 
tion is  an  element  of  little  value  in  determining  and  classi- 
fying the  rock,  compared  with  its  color,  texture,  structure, 
hardness  and  other  qualities. 

Chemical  Relationships.  The  chemical  and  mineral- 
ogical composition  of  sedimentary  rocks  is  not  dependent 
on  definite  laws,  as  that  of  the  igneous  rocks  evidently  is. 
There  are  no  rules  governing  the  associations  of  minerals, 
since  these  have  been  brought  together  by  chance,  depend- 
ing mostly  on  specific  gravity,  and  on  size  of  grain  in  the 
assortment.  The  chemical  composition  has  not  in  conse- 
quence the  same  significance  that  it  has  in  igneous  rocks. 
Analyses  of  a  few  of  the  more  important  types  are  given 
in  the  following  descriptive  portion,  since  these  may  be 
useful  in  several  ways. 

Classification  of  Sedimentary  Rocks.  Two  modes  are 
used  to  classify  the  stratified  rocks;  one,  without  reference 
to  composition  and  character,  is  based  upon  the  period 
of  their  formation  in  the  geological  time  scale;  the  other 
is  founded  on  composition  and  physical  characters. 
According  to  the  first,  strata  are  classified  as  Cambrian, 
Devonian,  Jurassic,  Tertiary,  etc.;  according  to  the 


CLASSIFICATION  OF  STRATIFIED  ROCKS         291 

second,  as  sandstones,  limestones,  etc.  The  first  has  its 
bearing  in  historical  geology,  the  second  is  the  petrological 
method,  and  is  the  one  that  concerns  us  here.  In  this 
work  the  following  classification  is  adopted. 

Classification  of  Stratified  Rocks. 

1.  Material  of  chemical  origin,  from  solutions. 

a.     Deposits  from  concentration. 

Sulphates;  GYPSUM  and  ANHYDRITE. 
Chlorides;  ROCK-SALT. 
Silica;  GEYSERITE  and  related  rocks. 
Carbonates;  TRAVERTINE  and  related  rocks. 
IRON  ORES  of  several  kinds. 

6.     Deposits  through  organic  life.* 

Carbonates;  LIMESTONE  and  DOLOMITE. 
Silica;  FLINT  and  related  rocks. 
Phosphate  rock. 
COAL,  asphalt,  etc. 

2.  Material  of  Mechanical  Origin. 

a.     Water-laid  deposits. 

CONGLOMERATES  and  BRECCIAS. 

SANDSTONES. 

SHALES. 

6.     Wind-formed  deposits. 
Loess. 

Dune-sands. 
Volcanic  ashes. 

c.     Surface  accumulations. 

Laterite  and  various  soils. 

In  the  nature  of  things  a  classification  of  stratified  rocks 
cannot  always  draw  exact  lines  between  different  kinds  of 
rocks.     For  shales  may  pass  into  limestones  on  the  one 
*  Geyserite,  Travertine,  and  Iron  Ore  may  be  also  partly  organic. 


292  ROCKS  AND.  ROCK  MINERALS 

hand,  and  into  sandstones  on  the  other,  and  no  sharply- 
defined  boundary  can  be  drawn  between  them.  Many 
such  instances  could  be  cited.*  And  in  cases  of  many 
rocks  of  mixed  materials  and  origin  it  would  be  difficult 
to  know  just  where  to  assign  them.  The  classification 
must  be  considered  as  based  upon  clear  and  unmistakable 
types,  which  serve  as  center  points  around  which  the 
rocks  group  themselves.  In  the  descriptive  portion 
which  follows,  the  exact  order  of  this  classification,  in 
respect  to  some  minor  rocks,  for  convenience  in  refer- 
ence, may  not  be  always  exactly  followed. 

*  Thus  geyserite  and  travertine  are  in  places  and  at  times  partly 
organic  in  origin. 


CHAPTER  IX. 
DESCRIPTION  OF  STRATIFIED  ROCKS. 

Chemical  Deposits  by  Concentration  and  Organic  Agencies. 

THE  more  important  of  the  deposits  produced  from 
aqueous  solutions  by  the  material  becoming  insoluble 
through  concentration  are  gypsum,  anhydrite,  rock-salt 
and  calcium  carbonate.  Certain  deposits  of  silica  from 
hot  waters  should  also  be  placed  here  and  iron  ores  as  well, 
although  in  the  latter  case  the  process  of  deposition  is  not 
usually  one  of  simple  concentration.  The  connection 
between  gypsum,  anhydrite  and  rock-salt,  in  respect  to 
their  origin  and  occurrence,  is  very  close.  They  are 
formed  in  bodies  of  sea-water  that  have  been  separated 
from  the  ocean  by  the  raising  of  coast-lines,  or  by  accumu- 
lations of  deposits,  and  under  such  climatic  conditions  that 
the  isolated  water  concentrates  by  evaporation  to  such  a 
degree  that  its  salts  must  crystallize  out  of  solution  and 
deposit.  Or  in  a  similar  way  they  may  be  formed  in 
inland  lakes,  which  have  no  outlet  because  they  are  in 
arid  regions,  where  the  evaporation  equals  or  exceeds 
the  amount  of  inflow.  All  natural  flowing  waters  contain 
more  or  less  of  various  salts  in  solution,  and  in  such 
a  lake  they  must  indefinitely  increase  until  the  deposit- 
ing point  of  concentration  is  reached. 

GYPSUM. 

As  a  rock,  gypsum  is  fine-grained  to  compact;  some- 
times a  foliated  aggregate  showing  the  excellent  cleavage 
of  the  mineral;  sometimes  it  has  a  fine  fibrous  structure; 
these  forms  are  less  common  than  the  first  one.  The  foli- 

293 


294  ROCKS  AND  ROCK  MINERALS 

ated  is  sometimes  cavernous  with  crystal  ends  projecting 
into  the  cavities,  and  this  may  be  from  recrystallization 
of  the  more  compact  varieties.  The  fibrous  variety  is 
more  apt  to  occur  when  gypsum  forms  thin  layers  or 
lenses  in  shales  and  sandstones.  The  usual  color  is  white, 
but  it  is  sometimes  yellow  or  red  from  iron  oxides,  or 
gray  to  dark  gray  from  mingled  clay  or  organic  matter. 
It  is  soft  and  easily  scratched  with  the  finger  nail.  For 
other  properties  reference  may  be  had  to  the  description 
of  gypsum  as  a  mineral. 

Gypsum  is  likely  to  be  accompanied  by  a  great  variety 
of  minerals  depending  on  the  local  occurrence.  The  most 
common  and  intimately  related  of  these  are  rock-salt  and 
anhydrite,  the  three  having  a  common  origin  as  previously 
stated.  Clay,  marl  and  bitumen  are  common  impurities. 
Dolomite,  calcite,  quartz,  sulphur,  iron  pyrites,  are  not 
uncommon  accessory  constituents.  Varieties  containing 
bituminous  substances  generally  yield  a  disagreeable  odor 
when  broken.  Gypsum  is  used  in  the  manufacture  of 
plaster  of  Paris,  and  in  the  raw  state  as  fertilizer.  The 
very  compact  white  or  tinted  varieties  are  sometimes 
called  alabaster,  and  cut  into  ornamental  forms,  vases,  etc. 

Occurrence.  Gypsum  is  widely  distributed  in  the  strat- 
ified rocks,  in  the  form  of  extensive  beds,  often  of  great 
thickness,  and  is  especially  associated  with  limestones  and 
snales.  It  is  very  commonly  found  accompanying  beds 
of  rock-salt;  in  such  cases  it  is  likely  to  underlie  the  salt. 
It  is  also  found  in  sedimentary  formations,  especially  in 
clays  and  shales,  in  lenticular  masses  or  scattered  through 
them  in  isolated  crystals,  sometimes  of  great  size,  as  in  the 
Cretaceous  beds  of  the  western  United  States. 

It  also  occurs  in  volcanic  regions,  around  fumaroles, 
where  sulphurous  vapors  are  escaping,  and  especially 
where  limestones  have  been  subjected  to  such  action.  In 
some  places  where  it  is  found  in  rocks  it  may  be  due  to  the 
oxidation  of  iron  pyrites  and  a  chemical  reaction  of  the 
product  with  carbonate  of  lime. 


DESCRIPTION  OF  STRATIFIED  ROCKS  295 


ANHYDRITE. 

As  a  rock,  anhydrite  is  a  compact  to  fine  granular  sub- 
stance; sometimes  coarse  and  showing  the  apparently 
cubic  cleavages  of  the  individual  grains.  It  may  be 
somewhat  translucent,  and  usually  has  a  somewhat 
splintery  fracture  with  a  shimmering  or  pearly  luster. 
Its  color  is  generally  white,  though,  like  gypsum,  it  is  often 
tinted  reddish,  yellowish,  bluish,  gray  or  dark  by  oxides 
of  iron,  or  commingled  clay,  or  organic  matter.  It  is 
harder  than  gypsum  but  easily  cut  with  a  knife.  For 
the  other  properties  see  description  of  it  as  a  min- 
eral. The  most  commonly  associated  minerals  are  rock- 
salt  and  gypsum,  but  locally  it  may  contain  many 
others,  as  those  stated  under  gypsum.  In  the  anhydrite 
beds  in  the  strongly  folded  regions  of  the  Alps,  the 
clay  impurity  has  been  converted  into  cyanite,  sillimanite, 
mica,  etc. 

The  occurrence  of  anhydrite  is  similar  to  that  of  gypsum, 
which  it  frequently  accompanies.  It  is  changed  on  ex- 
posure to  the  air  into  that  substance.  The  beds  do  not 
usually  show  any  distinct  stratification.  In  America, 
extensive  deposits  occur  in  Nova  Scotia. 

ROCK-SALT. 

This  is  an  aggregate  of  grains  of  common  salt,  halite 
or  sodium  chloride.  It  is  sometimes  fine,  sometimes 
medium,  and  sometimes  coarse  grained.  The  color  is 
white  but  it  is  often  red  or  yellowish  from  oxides  of  iron, 
gray  from  intermingled  clay  or  organic  matter,  and  the 
latter  may  at  times  produce  bluish  or  greenish  tints. 
The  properties  of  halite  are  mentioned  in  the  chapter  on 
rock  minerals. 

Associated  minerals  sometimes  found  in  the  salt  are 
quartz,  anhydrite  and  sometimes,  though  rarely,  carbonates 
or  pyrite. 

Rock-salt  occurs  in  geological  formations  of  the  sedi- 


296  ROCKS  AND  ROCK  MINERALS 

mentary  rocks  of  all  ages  and  in  many  parts  of  the  world, 
The  beds  vary  greatly  in  thickness,  from  one  foot  to  4000  or 
more.  Such  enormous  thicknesses  cannot  be  explained  by 
the  simple  concentration  of  an  isolated  body  of  sea-water 
along  an  arid  coast-line.  There  must  have  been  sub- 
sidence gradually  going  on;  at  first  the  less  soluble  gyp- 
sum, and  then  the  salt,  would  be  deposited,  leaving  a 
mother  liquor  containing  the  more  soluble  sulphates  and 
chlorides  of  magnesium  and  potassium.  If  subsidence 
and  the  lowering  of  the  barrier  should  now  occur,  there 
would  be  an  influx  of  the  lighter  sea-water  above,  while 
the  heavier  mother  liquor  would  flow  out  below  and  the 
basin  would  be  charged  anew  with  sea-water.  If  the 
barrier  is  again  closed,  for  example  by  waves  building  it  up 
as  seen  along  the  coast  of  the  Carolinas,  the  conditions 
would  be  repeated  and  fresh  deposits  of  gypsum  and  salt 
formed.  Thus  by  repetitions  of  such  a  process  we  can 
imagine  that  great  thicknesses  of  salt  might  be  locally 
deposited.  Finally,  if  no  outflows  occur  the  mother 
liquor  is  also  evaporated  and  the  more  soluble  salts 
deposited. 

In  the  United  States  rock-salt  occurs  in  beds  in  New 
York,  Michigan,  Louisiana,  Kansas  and  various  other 
states.  It  is  found  in  Europe,  in  Germany,  Austria  and 
Poland  in  vast  deposits;  in  several  counties  of  England  and 
in  many  other  places.  Interior  drainages  are  present  in 
all  of  the  continents  and  in  connection  with  them  there 
are  salt  lakes  and  deposits  of  salt. 

FLINT,  GEYSERITE  AND  OTHER  SILICEOUS  ROCKS. 

Deposition  of  silica,  Si02,  from  its  solution  in  water 
occurs  both  by  simple  concentration  and  evaporation 
and  by  the  action  of  organic  life.  It  is  possible  that  it 
may  also  happen  from  chemical  reactions.  The  deposits 
thus  formed,  while  lacking  the  wide  extent  and  geologic 
importance  of  the  sedimentary  formations  produced  by 
the  processes  of  erosion,  have  yet  considerable  interest 


DESCRIPTION  OF  STRATIFIED   ROCKS  297 

and  may  be  of  local  significance.  On  account  of  their 
general  similarity  of  composition  they  are  here  included 
under  one  heading,  but  the  group  does  not  include  the 
mechanically  formed  siliceous  sandstones.  The  material 
composing  these  rocks  is,  mineralogically,  sometimes  in  the 
form  of  quartz  —  pure  crystallized,  SiC>2,  and  sometimes 
in  the  form  of  opal  or  chalcedonic  silica  —  uncrystallized 
silica  containing  more  or  less  water  in  combination  as 
hydro  xyl. 

Flint.  This  is  a  dark  gray  or  black  rock,  so  extremely 
compact  that  it  appears  as  a  homogeneous  substance. 
The  fracture  is  conchoidal  and  the  chips  have  a  translucent 
edge  like  many  felsites,  which  indeed  it  may  closely 
resemble.  The  hardness  is  7.  It  consists  of  an  intimate 
mixture  of  quartz  and  opal,  the  coloring  matter  being 
organic  and  disappearing  when  a  chip  is  heated  before 
the  blowpipe. 

Flint  is  not  a  rock  in  the  sense  that  it  occurs  in  extended  inde- 
pendent formations.  It  occurs  in  irregular  nodules  or  concretions 
in  chalk  which  vary  widely  in  size,  from  that  of  a  pea  to  extensive 
layers.  Similarly  an  impure  flint,  occurring  chiefly  in  limestones 
from  the  Cambrian  up,  is  called  chert.  When  these  substances  are 
studied  under  the  microscope  they  are  found  to  contain  the  hard 
siliceous  parts  of  various  organisms,  chiefly  of  sponges  and  radio- 
larians.  The  matter  was  first  derived  from  sea-water  by  such 
organisms,  but  appears  secondarily  to  have  gone  into  solution  and 
been  chemically  deposited  around  certain  centers,  and  in  certain 
places,  where  favorable  conditions  obtained.  The  uses  of  flint  for 
savage  and  prehistoric  implements  and  weapons  and  for  striking 
fire  are  well  known.  Other  siliceous  masses,  similar  in  a  general 
way  to  flint  and  chert,  sometimes  of  the  same  and  sometimes  of 
uncertain  origin,  have  received  various  names  such  as  lydianite, 
harnstone,  etc.  Jasper  is  a  chemically  precipitated  opaline  silica. 
In  places,  as  in  the  Lake  Superior  region,  the  jaspers  are  strongly  fer- 
rugineous  and  interlaminated  with  bands  and  streaks  of  hematite. 
They  constitute  rock  masses  of  considerable  size,  affording  valuable 
deposits  of  iron  ore.  They  are  called  jaspilite.  The  cherty  layers 
are  colored  bright  red  by  the  iron  oxide.  Another  variety  of  these 
siliceous  flint-like  rocks  are  the  novaculites,  which  occur  in  con- 
siderable beds  in  Arkansas,  and  are  greatly  used  in  the  manu- 


298  ROCKS  AND   ROCK  MINERALS 

facturc  of  whetstones  and  hones.  They  are  very  dense,  conchoidal 
in  fracture,  white  or  pale  gray  in  color,  semi-translucent,  and  com- 
posed of  silica.  Their  origin  is  uncertain. 

Geyserite.  Siliceous  Sinter.  In  volcanic  regions  silica 
is  frequently  deposited  by  hot  waters,  boiling  springs  and 
geysers.  Sometimes  this  is  produced  by  simple  evapora- 
tion and  drying  of  the  water  and  sometimes,  as  shown 
by  Weed,  it  is  due  to  vegetable  organisms,  algce,  which 
secrete  silica  from  the  heated  waters  in  which  they  live 
and  become  coated  with  it.  The  material  of  the  geyser 
cones  and  basins  produced  by  drying  is  hard,  compact, 
and  opaline,  while  that  formed  by  the  plants  is  more  or 
less  loose,  spongy,  and  tufaceous.  If  pure,  it  is  white  in 
color.  Its  formation  is  well  illustrated  in  the  hot  spring 
and  geyser  areas  of  the  Yellowstone  Park  and  New  Zea- 
land (see  Plate  27).  The  material  thus  formed  is  known 
as  geyserite,  or  siliceous  sinter. 

Diatomaceous  Earth.  This  is  a  soft,  white,  chalk-like, 
very  light  rock  composed  of  innumerable  microscopic 
shells  of  diatoms.  The  latter  are  excessively  minute, 
unicelled  organisms  which  possess  free  motion  and  are 
covered  with  a  siliceous  shell  of  great  delicacy;  they  are 
considered  forms  of  vegetable  life.  In  waters  of  suitable 
character  they  may  swarm  in  incredible  numbers  and  their 
shells,  accumulating  at  the  bottom,  may  give  rise  to  de- 
posits of  considerable  magnitude.  Some  varieties  of  the 
rock  are  pale  yellow,  brown  or  gray.  It  is  easily  distin- 
guished from  chalk,  which  it  may  resemble,  by  its  non- 
effervescing  with  acid;  from  clay  by  its  gritty  feeling, 
when  rubbed  between  the  fingers,  and  its  weak  argil- 
laceous odor  or  the  absence  of  it.  A  more  positive  test  is 
the  effervescence  produced  when  it  is  mixed  with  car- 
bonate of  soda  and  fused  before  the  blow-pipe.  The 
loose,  scarcely  coherent  material  is  called  infusorial  earth; 
when  more  compact  it  is  sometimes  called  tripolite.  It 
is  extensively  used  for  polishing  purposes.  Beds  of  con- 
siderable magnitude  occur  in  the  United  States  in  Mary- 


PLATE  27. 


o" 

^  '& 

l-i  0 

PH  *0 

^  r^ 

02  O 

H  " 


DESCRIPTION  OF  STRATIFIED   ROCKS  299 

land,  Virginia,  Georgia  and  Alabama,  where  they  are 
worked  commercially,  also  Ln  Missouri,  Nevada,  California 
and  elsewhere,  often  as  a  layer  in  swamps  which  repre- 
sent the  fillings  of  former  lakes.  They  are  also  found  in 
Germany  and  other  parts  of  Europe. 

IRON  ORE  ROCKS. 

The  deposits  of  iron  ore  which  occur  as  rocks,  inter- 
stratified  or  associated  with  sedimentary  beds,  have  origi- 
nated through  complex  processes,  sometimes  wholly, 
sometimes  partly,  of  a  purely  chemical  nature  and  usually 
more  or  less  influenced  by  the  agencies  of  organic  life. 
The  most  important  set  of  processes  has  been  previously 
mentioned  but  now  deserves  a  more  detailed  description. 

Iron  exists  in  the  original  (the  igneous)  rocks  in  the 
form  of  silicates,  such  as  biotite,  olivine,  pyroxene  and 
hornblende,  and  also  as  oxides,  such  as  magnetite,  hema- 
tite and  ilmenite,  as  disseminated  grains.  It  also  occurs 
in  the  secondary  metamorphic  rocks  as  silicates  and  oxides. 
It  is  also  pretty  generally  diffused  through  the  sedimentary 
rocks,  in  part  as  coloring  matter  and  cement,  and  mostly 
in  the  form  of  ferric  oxide,  ferric  hydroxides  and  ferrous 
carbonate.  In  the  igneous  rocks  it  is  largely  in  the  ferrous 
state  and  to  a  considerable  degree  also  in  the  meta- 
morphic ones.  Also,  to  understand  the  concentration  of 
iron  and  formation  of  iron  ore  rocks,  it  must  be  borne  in 
mind  that  the  metal  forms  only  one  carbonate,  ferrous 
carbonate  or  siderite,  FeCOs  which,  like  carbonate  of 
lime,  is  soluble  in  water  containing  carbon  dioxide. 

When  the  rocks  are  decomposed  and  broken  down  by 
the  agencies  of  weathering  and  erosion,  the  silicates  con- 
taining iron  are  altered;  the  ferrous  oxide  in  them  com- 
bines in  part  with  the  carbon  dioxide  in  the  circulating 
ground  water  to  form  ferrous  carbonate  which  goes  into 
solution,  and  in  part  it  is  oxidized  to  ferric  oxide.  The 
original  oxides  of  iron  react  in  a  similar  manner.  The 
ferric  oxide  thus  formed  or  liberated  would  be  insoluble, 


300  ROCKS  AND  ROCK  MINERALS 

but  in  the  presence  of  decaying  vegetable  matter  in  the 
soil  and  organic  acids  leached  downward  into  the  rocks, 
deoxidation  of  the  ferric  oxide  ensues;  it  is  reduced  to 
ferrous  oxide  and  then  becomes  ferrous  carbonate  and 
goes  into  solution.  The  reason  for  this  is  that  decay  of 
organic  matter  is  a  process  of  oxidation,  like  slow  com- 
bustion; the  organic  matter  takes  oxygen  from  the  air, 
but  in  the  presence  of  moisture  and  ferric  oxide  it  will 
take  oxygen  from  the  latter,  reducing  it  to  the  ferrous 
oxide  which  is  then  fitted  to  unite  with  carbon  dioxide 
and  become  the  carbonate. 

The  iron  of  the  rocks,  which  is  thus  brought  into  solu- 
tion, is  leached  out,  and  in  standing  bodies  of  shallow  water, 
such  as  swamps,  lagoons  or  estuaries,  with  small  outlets 
to  the  sea,  it  may  be  concentrated  and  give  rise  to  exten- 
sive deposits.  Under  some  conditions  these  deposits  may 
be  of  the  carbonate  directly,  but  usually  the  solution  of 
the  carbonate  is  re-oxidized,  carbon  dioxide  escapes,  and 
the  iron  is  precipitated  as  ferric  hydroxide  (limonite). 
This  oxidation  is  largely,  if  not  wholly,  performed  by  cer- 
tain bacterial  organisms  which  demand  iron  in  their  inter- 
nal economy,  and  therefore,  secrete  the  iron  from  the 
water,  and  change  it  in  their  cells  from  the  ferrous  to  the 
ferric  condition,  thus  rendering  it  insoluble.  Living  and 
dying  in  unimaginable  numbers,  though  excessively 
minute,  they  give  rise  to  large  deposits. 

The  ferric  hydroxide  which  is  thus  precipitated  may 
accumulate  on  the  bottom  as  bog  iron  ore,  or  limonite,  or, 
as  is  so  often  the  case  in  shallow  bodies  of  standing  water, 
like  swamps,  etc.,  it  may  again  come  in  contact  with  decay- 
ing vegetable  matter,  and  be  changed  back  into  carbonate. 
Such  beds  of  iron  ore  may  be  quite  pure,  or  they  may  be 
more  or  less  mingled  with  clay  and  sand,  brought  in  at 
times  of  high  water,  and  thus  impure  limonites,  clay  iron- 
stones, black-band  ore,  etc.,  are  formed.  This  also 
explains  the  not  infrequent  association  of  stratified  iron 
ore  and  coal  beds  in  the  same  series  of  rocks,  and  the  reason 


DESCRIPTION  OF  STRATIFIED  ROCKS  301 

why  in  this  case  the  iron  ore  is  commonly  ferrous 
carbonate. 

The  moving  ground  waters  containing  iron  in  solution, 
as  described  above,  may  also  issue  as  springs  and  give  rise 
to  deposits  of  iron  ore. 

Certain  masses  of  iron  ore,  chiefly  limonite,  are  supposed 
to  be  residual  products  of  weathering  and  solution.  This 
is  illustrated  in  the  view  that  masses  of  limestone  con- 
taining ferrous  carbonate  have  been  dissolved  and  carried 
away,  but  the  iron,  oxidized  to  the  ferric  condition  in  the 
process,  has  become  insoluble  and  remaining  behind  has 
gradually  concentrated.  The  more  important  iron  ore 
rocks  may  now  be  described. 

Bog  Iron  Ore.  Limonite.  This  is  sometimes  loose  and 
earthy,  sometimes  firm  and  porous.  It  consists  mainly 
of  limonite,  mixed  more  or  less  with  humus,  phosphates, 
silicates  of  iron,  clay,  sand,  etc.  Its  character  has  been 
sufficiently  described  under  limonite  among  the  minerals. 
It  sometimes  occurs  in  concretions.  With  increasing 
amounts  of  clay  it  passes  over  into  yellow  ocher.  It  is 
found  in  all  parts  of  the  world.  In  the  United  States  it 
is  widely  distributed,  and  along  the  Appalachian  belt, 
from  Vermont  to  Alabama,  deposits  of  limonite,  most  of 
which  are  probably  residual  in  character,  have  furnished 
iron  ore  since  the  early  settlement  of  the  country,  and  in 
great  quantity. 

Clay  Ironstone.  Siderite.  When  reasonably  pure,  side- 
rite,  or  spathic  iron  ore,  is  a  coarse  to  fine  crystalline  aggre- 
gate of  siderite  grains.  It  is  whitish  to  yellow,  or  pale 
brown  in  color,  but  on  exposed  surfaces  much  darker 
brown  to  black,  owing  to  oxidation  of  the  ferrous  carbon- 
ate to  limonite,  or  of  the  manganous  carbonate  to  manganic 
oxides.  It  generally  contains,  more  or  less,  carbon- 
ates of  lime,  magnesia  and  manganese.  Iron  pyrites  or 
hematite  are  commonly  associated  minerals.  For  the 
properties  of  siderite,  reference  may  be  had  to  its  de- 
scription among  the  minerals. 


302  ROCKS  AND  ROCK  MINERALS 

An  impure  variety  of  siderite  mixed  with  clay,  sand 
and  limonite  in  variable  proportions,  of  a  compact  appear- 
ance, and  generally  of  dull  brown  colors,  is  known  as  clay 
ironstone.  It  is  apt  to  occur  in  nodules,  often  as  con- 
cretions around  some  fossil,  and  lenticular  masses  which 
increase  until  they  become  interstratified  beds  of  consid- 
erable thickness.  Another  variety  which  contains  so 
much  organic,  coaly  matter  that  it  is  colored  black  is 
known  as  black-band  ore.  It  is  especially  associated 
in  the  strata  with  coal  beds  from  the  Carboniferous 
upward. 

Carbonate  ores  of  iron  are  of  less  importance  in  the 
United  States  than  the  deposits  of  limonite  and  hematite. 
They  occur  in  Pennsylvania,  Ohio  and  Kentucky,  of  Car- 
boniferous age,  and  in  the  Lake  Superior  region  in  Michigan 
and  Minnesota,  of  Algonkian  age.  They  occur  in  Europe 
in  England,  Germany,  France  and  Spain,  in  deposits  of 
great  technical  value.  Black-band  is  found  in  the  coal- 
bearing  strata  of  Pennsylvania,  England,  etc. 

Hematite.  Red  Iron  Ore.  This  occurs  in  the  form  of 
veins,  lenticular  masses  and  beds,  in  various  geological 
formations  and  especially  in  those  whose  strata  have  been 
folded.  As  a  rock,  it  varies  from  fine  grained  and  compact 
to  earthy  or  fibrous,  is  of  a  red  to  brown  color  or,  where 
crystalline,  of  a  dark  gray.  Its  properties  as  a  mineral 
have  been  previously  given.  It  occurs  pure  or  nearly  so, 
but  with  varying  mixtures  of  clay,  sand  or  silica,  it  passes 
insensibly  into  ferrugineous  clays,  red  ochers,  or  shales, 
sandstones,  cherts,  etc.  In  this  connection  see  jaspilite 
under  flint.  While  hematite  undoubtedly  occurs  as  a 
normal  sedimentary  or  stratified  rock,  interbedded  with 
other  unchanged  strata,  as  in  the  beds  which  have  such 
a  wide  distribution  in  the  eastern  United  States  in  the 
Clinton  group  of  the  Niagara  period,  it  is  more  generally 
to  be  considered  a  metamorphic  rock,  and  as  such,  might 
be  included  among  the  metamorphic  iron  rocks  described 
in  Chapter  XI,  such  as  itabirite  and  hematite  schists. 


DESCRIPTION  OF  STRATIFIED  ROCKS  303 

Extensive  deposits  of  hematite  are  found  in  various 
parts  of  the  United  States  and  Canada.  The  greatest 
amounts  mined  as  ore  come  from  Tennessee  and  the  Lake 
Superior  Region,  the  vast  production  in  the  latter  leading 
the  world  in  output.  Large  beds  are  also  found  in  England 
and  other  parts  of  Europe. 

Iron  Oolite.  The  iron  rocks  described  above,  and  especially  red 
hematite,  not  infrequently  assume  a  concretionary  form  in  which 
the  rock  is  composed  of  rounded,  sometimes  polygonal,  grains  which 
vary  in  size  from  that  of  fine  sand  to  peas.  An  examination  of 
them  shows  that  they  have  a  concentric  shelly  structure.  The  color 
varies  from  red  to  brown.  Sometimes  the  rock  is  composed  of 
them  alone  and  sometimes  they  are  thickly  embedded  in  a  marly  or 
clayey  cement.  The  iron  ore  appears  in  many  cases  to  have  been 
deposited  around  grains  of  sand,  fragments  of  fossils,  etc.,  as  neuclei. 
The  Clinton  ores  mentioned  above  frequently  assume  this  oolitic 
character  and  it  is  well  known  from  various  European  localities. 
Such  ores  have  sometimes  been  changed  into  magnetite  while  still 
retaining  the  oolitic  structure. 

LIMESTONE    AND  OTHER   CARBONATE   ROCKS. 

This  group  of  rocks  has  the  common  property  of  being 
composed  of  carbonate  of  lime,  calcite,  CaCOs,  or  of 
this  substance  intermingled  more  or  less  with  dolomite, 
MgCa (003)2-  It  is  also  a  common  property  that,  so  far 
as  known,  the  carbonate  of  lime  has  primarily  been 
separated  from  water,  rendered  insoluble  and  accumu- 
lated by  the  action  of  living  organisms  of  one  kind  or 
another.  Secondarily,  the  deposits  thus  made  may  be 
mechanically  broken  up  and  redeposited,  or  they  may  be 
taken  into  solution,  carried  away  and  precipitated  else- 
where. There  may  be  some  possible  exceptions  to  this 
rule,  that  the  carbonate  of  lime  is  primarily  precipitated 
by  organisms,  in  the  cases  where  it  is  concentrated  in 
alkaline  lakes  by  inflowing  waters  and  finally  deposited, 
or  in  the  evaporation  of  shut-off  portions  of  the  sea,  but 
these  are  of  small  account  and  negligible  in  comparison 
with  the  great  formations  produced  by  life  agencies. 


304  ROCKS  AND  ROCK  MINERALS 

Hence  it  is  generally  held  that  the  great  masses  of  car- 
bonate rocks,  even  when  they  do  not  contain  fossils, 
are  a  proof  of  the  existence  of  life  at  the  time  of  their 
original  deposition. 

This  group  of  rocks  is  soluble  in  hydrochloric  acid; 
entirely  so  when  pure  carbonates,  but  generally  leaving 
more  or  less  of  a  residue,  consisting  chiefly  of  sand,  clay, 
silica,  etc.  In  some  cases,  where  dolomite  is  present,  the 
acid  must  be  heated.  Their  hardness  is  less  than  4, 
hence  they  may  be  readily  scratched  or  cut  with  the 
knife. 

The  following  are  the  important  members  of  the  group. 

Limestone.  This  is  the  most  common  and  important 
carbonate  rock.  It  is  fine  grained  to  very  dense  in 
texture  and  its  color  varies  from  whitish,  through  tones  of 
yellowish  to  brown,  or  from  various  shades  of  gray,  dove- 
color,  bluish-gray,  dark-gray  to  black.  It  is  rarely  of 
reddish  colors.  The  yellow  and  brown  colors  are  due  to 
iron  oxide,  the  gray  and  black  to  organic  matter.  The 
gray  colors  are  most  common.  Compact  varieties  have 
an  even  to  somewhat  conchoidal  fracture.  It  effervesces 
freely  with  any  common  acid,  with  vinegar  (acetic  acid) 
or  lemon  juice  (citric  acid).  It  is  easily  scratched  with 
the  knife  and  many  of  the  less  compact  varieties  are 
friable  to  the  finger  nail.  The  specific  gravity  varies 
from  2.G-2.8.  On  exposed  surfaces  it  is  apt  to  be  cavern- 
ous and  often  tinted  or  blotched  reddish  or  yellowish  from 
oxidation  of  small  amounts  of  ferrous  carbonate  it  may 
contain.  It  occurs  in  individual  beds  of  all  thicknesses 
up  to  100  feet  or  more. 

Some  limestones  consist  of  pure  grains  of  calcite,  others 
possess  a  fine,  clay-like  cement  between  them.  Acces- 
sory minerals,  which  are  sometimes  seen,  are  pyrite  and 
quartz,  the  latter  in  minute  crystals,  sometimes  lining 
cavities. 

In  following  analyses  I,  II,  and  III  are  of  very  pure 
limestones;  IV  is  an  impure  type  containing  considerable 


DESCRIPTION  OF  STRATIFIED   ROCKS 


305 


dolomite  and  sand  and  clay.  Such  transitions  through 
impurities  are  common;  thus  V  for  example  shows  one 
toward  the  clay-ironstone  previously  described.  Transi- 
tions to  dolomite  are  not  common;  an  examination  of  a 
large  number  of  analyses  shows  that  generally  either  the 
rock  contains  very  little  or  no  magnesia,  or  it  has  much 
and  is  a  regular  dolomite  as  described  later. 


SiO2 

A12O3 

Fe2O3 

FeO 

MgO 

CaO 

H2O 

XyO 

C02 

Total 

I.... 

0.4 

_ 

0.4 

_ 

trace 

54.8 

1.1 

_ 

42.7 

99.4 

II... 

0.6 

— 

0.4 

— 

0.4 

54.2 

— 

_ 

44.0 

99.6 

III.. 

1.2 

0.2 

0.2 

0.3 

0.6 

53.8 

0.9 

0.1 

42.7 

100.0 

IV.. 

7.0 

3.6 



9.0 

39.3 

0.2 

1.2 

38.8 

99.1 

V... 

3.2 

0.1 

10.9 

15.2 

11.0 

26.6 

0.5 

1.5 

41.1 

100.1 

I,  Trenton  Limestone,  Lexington,  Virginia  ;  II,  Buff  Limestone, 
Hoosier  Quarry,  Bedford,  Indiana;  III,  Lithographic  Limestone, 
Solenhofen,  Bavaria;  IV,  Impure  dolomitic  Limestone,  Greason, 
Pennsylvania;  V,  Sideritic-dolomitic  Limestone,  Gogebic  dist., 
Michigan. 

XyO  represents  small  quantities  of  organic  matter,  manganese 
oxide,  etc. 

The  strength  of  limestone  as  a  rock  varies  very  much 
with  the  texture;  that  of  firm  compact  varieties  is  very 
high  while  loose  porous  ones  are  very  weak.  Thus  a 
dense  variety  has  been  shown  to  have  a  crushing  strength 
of  over  40,000  pounds  per  square  inch,  while  others 
scarcely  exceed  3000  pounds  per  square  inch.  The  well 
known  white  oolitic  limestone  of  Bedford,  Indiana,  has  an 
average  crushing  strength  of  4300  pounds.  Any  good 
firm  and  compact  limestone  has  a  strength  far  in  excess  of 
any  load  that  it  may  be  called  upon  to  endure  in  modern 
structures.  The  porosity  of  limestones  varies  considerably; 
those  containing  the  most  sand  are  usually  the  most 
porous;  the  ratio  of  pore  space  to  rock  volume  may  vary 


306  ROCKS  AND  ROCK  MINERALS 

from  15  per  cent  to  practically  nothing,  the  ratio  of  the 
weight  of  water  it  can  absorb  to  the  weight  of  rock  is  in 
general  much  less  than  this,  usually  not  more  than  one- 
half  as  much. 

There  are  many  varieties  of  limestone,  depending  on  circumstances, 
especially  the  mode  of  formation.  Thus  in  some  there  are  abun- 
dant remains  of  fossils  which  may  give  the  rock  a  distinctive  char- 
acter. These  comprise  a  great  variety  of  organisms,  among  which 
may  be  mentioned  corals,  crinoids,  shells  of  mollusks,  brachiopods, 
gastropods,  foraminifera,  remains  of  sponges,  etc.  The  "  encrinal 
limestone  "  of  Silurian  age  in  western  New  York  is  an  example. 
Sometimes  these  fossils  occur  in  such  numbers  that  the  entire  rock 
is  composed  of  masses  of  shells,  or  the  hard  part  of  one  particular 
organism,  with  just  enough  fine  carbonate  of  lime  between  them 
to  act  as  a  cement.  Examples  of  this  are  seen  in  the  layers  com- 
posed wholly  of  brachiopod  shells  found  in  the  Niagara  formation 
of  the  Silurian  in  western  New  York.  Such  rocks  are  sometimes 
called  "shell  limestones."  Certain  limestones  composed  of  corals 
are  also  examples  of  the  same  thing. 

On  the  other  hand,  there  are  varieties  which  depend  on  the  presence 
of  some  impurity  which  gives  a  particular  character  to  the  rock. 
Thus  it  may  contain  much  clay  and  is  termed  an  argillaceous  lime- 
stone or  it  may  contain  much  sand  of  siliceous  character  and  be  an 
arenaceous  limestone :  such  rocks  are  transitional  to  shales  and 
sandstones.  Others  which  are  dark  colored  may  yield  a  strong, 
disagreeable,  bituminous  odor  when  struck  and  broken  and  are 
called  bituminous  limestones;  they  contain  considerable  organic 
matter.  In  some,  which  are  termed  glauconitic  limestone,  the  rock  is 
more  or  less  rilled  with  green  grains  of  glauconite.  Lithographic 
stone  is  a  fine,  compact,  somewhat  schistose  limestone;  the  flesh- 
colored  rock  from  Solenhofen,  Bavaria,  remarkable  for  the  well 
preserved  fossils  it  occasionally  contains,  is  especially  used  for  this 
purpose.  It  is  a  very  pure  limestone,  as  shown  by  the  analysis 
given  above. 

Limestones  are  very  apt  to  contain  concretions  and  masses  of 
chert,  or  hornstone,  of  the  character  described  in  a  previous  section ; 
they  often  become  so  abundant  as  to  form  definite  bands  or  layers 
in  the  rock. 

By  the  weathering  of  limestones  the  lime  carbonate  is 
removed  in  solution,  leaving  the  insoluble  impurities 
behind.  These  form  clays  or  loams  which  are  colored 


307 


deeply  red  or  yellow  by  the  oxidation  of  the  iron  min- 
erals originally  present,  and  commonly  contain  pebbles 
of  chert  or  quartzose  material  and  masses  of  limonite. 
Such  residual  soils  are  commonly  very  fertile  and  cover 
large  areas  in  the  southern  United  States,  and  in  other 
parts  of  the  world. 

Uses  of  Limestone.  The  use  of  limestone  for  structural 
purposes  of  all  kinds  is  well  known  and  needs  no  further 
comment.  The  same  is  true  of  its  manufacture,  by  burn- 
ing, into  quicklime  for  mortar  and  cements.  Large 
quantities  are  also  used  as  a  flux  in  smelting  operations,  as 
in  the  making  of  iron  and  steel.  In  recent  years  the  use  of 
certain  impure  limestones  containing  15-40  per  cent  of 
clay,  or  other  substances  consisting  of  silica,  alumina  and 
iron  oxide,  in  the  manufacture  of  natural  hydraulic 
cements  has  risen  to  very  large  proportions. 

Dolomite.  The  geological  use  of  this  term  is  not  always 
the  same  as  the  chemical  one.  Chemically,  or  mineralogic- 
ally,  by  dolomite  is  meant  a  chemical  compound  of  a 
definite  composition  CaMg(COs)2  with  CaO,  30.4  per 
cent,  MgO  21.7,  CO2  47.8,  while  geologically  the  term  is 
used  for  any  limestone  which  consists  dominantly  of 
this  compound,  although  it  may  also  contain  a  large 
amount  of  admixed  calcite,  CaCOs,  and  in  some  parts  of 
Europe  it  is  employed  to  designate  limestones  of  a  particu- 
lar geological  period,  some  of  which  are  not  dolomites 
at  all. 

The  description  of  the  colors,  texture,  and  other  physical 
characters  of  limestone  given  above,  applies  equally  well 
to  dolomite.  In  fact  it  cannot  ordinarily  be  told  in  the 
field,  or  by  mere  inspection  of  a  hand  specimen  of  a  rock, 
whether  it  is  a  dolomite  or  a  pure  limestone. 

Dolomite  is  somewhat  harder  than  true  limestone  and 
if  it  is  a  pure  dolomite  it  will  not  dissolve  with  efferves- 
cence in  acetic  acid  (vinegar)  and  but  very  slowly  in  cold 
hydrochloric;  if  it  contains  admixed  calcite  this  reacts 
very  readily.  The  best  test  is  a  chemical  one  for  magnesia 


308 


ROCKS  AND   ROCK  MINERALS 


in  a  solution  obtained  by  boiling  the  powdered  rock  in 
dilute  hydrochloric  acid. 

The  following  analyses  show  the  chemical  composition 
of  some  examples  of  this  rock. 


Si02 

A1203 

Fe2O3 

FeO 

MgO 

CaO 

CO2 

H2O 

XyO 

Total 

I.... 

3.2 

0.2 

0.2 

0.1 

20.8 

29.6 

45.5 

0.3 

99.9 

II... 

3.1 

_ 

0.1 

0.9 

20.0 

29.7 

45.3 

0.3 

0.2 

99.6 

III.. 

1.1 

0.4 

— 

— 

19.9 

31.5 

45.6 

1.3 

0.1 

99.9 

IV.. 

5.0 

1.0 

0.5 

— 

16.8 

32.2 

43.8 

— 

— 

99.3 

I,  Knox  Dolomite,  Morrisville,  Alabama;  II,  Dolomite,  Sunday 
Lake,  Gogebic  district,  Michigan;  III,  Dolomite,  Tornado  Mine, 
Black  Hills,  South  Dakota;  IV,  Dolomite  (magnesian  limestone), 
Newcastle,  England. 


The  origin  of  dolomite  is  a  matter  which  has  been  much  discussed 
and  many  theories  have  been  propounded  by  geologists  and 
chemists  in  explanation  of  it.  When  all  the  facts  are  taken  into  con- 
sideration, it  is  clear  that  dolomite  is  not  an  original  rock,  but  has 
been  formed  from  pure  limestones  by  the  substitution  of  a  part  of 
the  lime  by  magnesia,  from  waters  containing  magnesium  salts  in 
solution.  Dolomite  is  a  denser  and  more  stable  compound  than 
calcite;  if  the  latter  were  subjected  to  the  action  of  soluble  magne- 
sium salts  there  would  be  a  constant  tendency  for  dolomite  to  form 
and  part  of  the  lime  to  be  liberated,  as  illustrated  in  the  following 
reaction, 

2  CaCO3  +  MgCl2  =  CaMg  (CO3)2  +  CaCl2. 

It  is  evident  that  if  the  magnesia  solution  was  strongly  concen- 
trated the  exchange,  in  a  given  mass  of  limestone,  would  be  effected 
more  quickly.  If  the  solution  were  heated  it  would  also  act  more 
quickly;  if  it  acted  under  pressure  the  result  would  also  be  hastened, 
and,  finally,  as  time  is  an  important  element,  the  longer  the  limestones 
have  been  subjected  to  the  solutions  the  more  completely  we  may 
expect  them  to  be  changed  to  dolomite.  If  we  consider  in  addition, 
that  not  only  sea- water  contains  magnesium  salts,  but  also  the  circu- 
lating ground  waters  and  thermal  waters  ascending  from  the  depths, 
in  greater  or  lesser  amount,  it  is  clear  that  in  harmony  with  the 


DESCRIPTION  OF  STRATIFIED  ROCKS  309 

above  principles,  the  change,  which  we  may  call  dolomitization,  must 
take  place  in  a  variety  of  ways  and  under  various  conditions,  not 
only  in  the  sea,  but  also  on  the  land;  that  all  limestones  are  not 
converted  completely  into  dolomite  before  they  emerge  from  the  sea 
must  be  due  to  certain  reasons;  that  the  solution  is  too  dilute,  that 
it  is  not  hot  enough,  that  there  has  not  been  sufficient  time,  that  the 
deposits  are  too  compact  to  permit  sufficient  penetration  and  cir- 
culation of  sea-water,  etc.  But  if  lime  deposits  are  subjected  in 
an  enclosed  basin  to  constantly  concentrating  sea-water  they  may 
become  more  rapidly  converted.  This  might  happen,  for  instance, 
if  a  coral  atoll  were  somewhat  elevated  and  its  lagoon  wholly  or 
nearly  shut  off  from  access  to  the  sea.  The  formation  of  dolomite 
in  such  enclosed  basins  of  sea-water  would  also  explain  its  frequent 
association  with  gypsum  and  anhydrite.  The  application  of  the 
principles  stated  above  would  also  lead  us  to  conclude,  that  the 
older  and  more  deeply  buried  a  limestone  was,  the  more  apt  it  would 
be  to  become  a  dolomite;  that  in  disturbed  and  folded  mountain 
regions,  limestones  of  the  same  age  and  formation  would  be  more 
likely  to  be  dolomitic  than  those  of  undisturbed  areas,  because  the 
rocks  are  there  more  fractured  and  filled  with  thermal  solutions,  and 
in  practice  the  facts  are  found  to  confirm  these  views.  The  connec- 
tion with  thermal  waters  also  explains  the  frequent  association  with 
lead  and  zinc  ores. 

The  mineral  dolomite  is  denser  than  calcite  and  in  the  change 
above  mentioned  a  considerable  reduction  of  volume,  amounting 
to  12  per  cent,  must  occur  in  the  limestone.  This  would  apparently 
help  to  explain  why  dolomites  are  so  frequently  very  porous  or 
cavernous  rocks,  though  if  deeply  buried,  all  such  cavities  would  be 
closed  by  the  pressure. 

Limestones  and  dolomites  are  rocks  of  such  general 
distribution  in  all  parts  of  the  world  where  stratified 
rocks  are  found,  that  their  occurrence  needs  no  special 
mention. 

Oolite.  Oolitic  Limestone.  This  is  a  well-characterized 
variety  of  limestone,  which  consists  of  minute  to  small 
spherical  concretions,  presenting  very  much  the  aspect  of 
a  fish  roe,  whence  the  name  from  the  Greek,  meaning  egg- 
stone.  The  round  grains  vary  in  size  from  very  minute 
up  to  those  as  large  as  a  pea.  In  the  larger  ones  it  may  often 
be  observed  that  they  have  a  concentric  shelly  structure 
and  thus  consist  of  successive  coats.  An  illustration  of  a 


310  ROCKS  AND  ROCK  MINERALS 

coarse  oolite  or  pisolitic  limestone  from  Bohemia  is  shown 
on  Plate  28.  There  is  usually  more  or  less  limy  cement 
binding  the  grains  together. 

Examination  of  oolites  generally  shows  that  some  object,  such  as 
a  bit  of  shell,  a  grain  of  sand  or  something  similar,  has  served  as  a 
nucleus  around  which  the  coatings  of  lime  carbonate  have  accumu- 
lated. On  the  shores  of  Great  Salt  Lake  at  the  present  time  oolitic 
sands  are  forming  from  the  waters  which  are  charged  with  lime  and 
other  salts  in  solution.  As  the  particles  are  rolled  on  the  beach,  or 
agitated  in  the  water,  all  parts  become  equally  coated  and  the 
spherical  form  is  assumed.  By  a  similar  process  oolitic  grains  are 
forming  in  springs  charged  with  lime  salts,  as  at  Carlsbad  in  Bohemia. 
The  concretionary  structure  is  best  seen  under  the  microscope; 
it  is  rarely  sufficiently  coarse  to  be  observed  with  the  eye  alone,  but 
may  be  sometimes  made  out  with  a  lens.  Oolitic  limestones  con- 
stitute large  and  important  formations,  often  of  great  thickness  and 
of  different  geological  ages.  They  are  especially  important  in  the 
Jurassic  strata  of  England  and  elsewhere  in  Europe.  It  is  a 
structure  also  assumed  by  some  American  limestones. 

Chalk.  Typical  chalk  is  a  soft,  white,  friable  rock,  whose 
use  for  marking  and  blackboard  crayons  is  well  known. 
While  generally  pure  white  it  may  sometimes  be  tinted 
gray,  flesh  color,  or  buff.  It  consists  of  a  fine  calcareous 
powder,  which  by  examination  under  the  microscope  has 
been  found  to  consist  of  the  tiny  shells  of  foraminifera, 
mingled  with  minute  fragments  of  the  shells  and  hard 
parts  of  various  organisms,  as  well  as  the  siliceous  spicules 
of  sponges,  shells  of  diatoms  and  radiolarians,  together 
with  occasional  microscopic  fragments  of  various  min- 
erals. It  is  the  siliceous  material  of  the  sponge  spicules, 
diatom  shells,  etc.,  that  has  concentrated  into  the 
nodules  and  concretions  of  flint,  so  commonly  found  in 
certain  beds  of  chalk,  and  whose  analogue  is  seen  in 
the  layers  and  masses  of  chert  in  limestones.  Chalks, 
in  spite  of  their  fine  grain,  are  very  porous  rocks,  absorb- 
ing as  much  water  as  20  per  cent  of  their  weight  in  some 
cases. 

Chemically,  chalks  are  quite  pure  carbonate  of  lime,  as 


I 


A.   OOLITE,    VARIETY   PISOLITE,    BOHEMIA. 


B.   COgUIXA,    FLORIDA. 


DESCRIPTION  OF  STRATIFIED  ROCKS 


311 


shown  in  the  following  analyses  of  three  specimens  given 
by  different  authorities. 

It  has  been  customary  to  consider  chalk  a  formation  produced  on 
the  bottom  of  the  deep  sea,  from  its  resemblance  to  the  calcareous 
oozes  or  muds  found  underlying  the  depths  of  modern  oceans.  It 
has  evidently  not  always  been  formed  in  this  way,  as  shown  by  the 
fossils  indicative  of  shallow  water  which  some  chalks  contain,  as  well 
as  the  perfect  skeletons  of  birds,  pterosaurs  and  other  vertebrate  ani- 
mals. The  facts  in  some  cases  would  point  rather  to  its  having  been 
formed  in  clear,  warm  and  shallow  seas,  free  from  the  products  of 
land  waste. 

Closely  related  to  chalk,  but  differing  in  the  fact  that  they  do  not 
predominantly  consist  of  foraminiferal  shells,  are  light,  chalky,  earthy 
limestones  formed  in  a  variety  of  ways,  such  as  from  coral  sands  and 
muds;  from  those  materials  accumulated  by  the  wind  on  coral 
islands ;  from  ground-up  shells  in  clear,  shallow  seas,  etc.  A  whitish, 
fragile  rock  formed  on  the  coasts  of  Florida,  which  consists  of  shells 
and  their  fragments  of  all  sizes  somewhat  lightly  compressed  and 
cemented  together,  is  known  as  Coquina,  from  the  Spanish  word  for 
shell.  (See  Plate  28.) 


CaCO3 

MgCO3 

SiO2 

(FeAl)203 

H2O 

Total 

I.. 

94.2 

1.4 

3.5 

1.4 

0.5 

101.0 

II.... 

96.4 

1.4 

1.6 

0.4 

0.2 

100.0 

III... 

98.4 

0.1 

1.1 

0.4 

~~ 

100.0 

I,  White  chalk,  White  Cliffs,  Little  River,  Arkansas;  II,  Lower 
Cretaceous  chalk,  Burnet  Co.,  Texas;  III.,  White  chalk,  Shore- 
ham,  Sussex  Co.,  England. 


Chalk  is  found  extensively  in  Europe  in  England, 
Germany,  France,  etc.,  where  its  occurrence  is  the  result 
of  a  distinct  geologic  epoch,  named  on  this  account 
the  Cretaceous.  It  also  occurs  widely  distributed  in 
the  Cretaceous  formations  of  the  southern  trans-Missis- 
sippi States,  in  Nebraska,  Arkansas,  and  especially  in 
Texas. 


312  ROCKS  AND   ROCK  MINERALS 

Travertine,  Calcareous  Tufa.  In  the  preceding  chaptei 
it  was  shown  how  material  of  the  land  surface  is  taken  into 
solution  and  carried  into  the  sea.  This  is  especially 
important  with  regard  to  lime,  which  goes  to  the  sea  as  a 
sulphate  and  carbonate,  the  latter  being  much  the  more 
momentous.  This  lime  carbonate  comes,  not  only  from 
pre-existent  carbonate  rocks,  but  also  from  the  lime  sili- 
cate minerals  of  the  igneous  and  metamorphic  ones,  which 
under  atmospheric  agencies  are  converted  into  carbonates. 
The  lime  carbonate  on  its  way  to  the  sea  may  be  tem- 
porarily deposited,  giving  rise  to  rock-masses  of  some 
magnitude  and  importance. 

Carbonate  of  lime  has  little  solubility  in  pure  water,  but  if  the  latter 
contains  carbon  dioxide,  the  lime  carbonate  is  converted  into  a  soluble 
bicarbonate  and  the  amount  of  the  latter  formed  and  taken  into 
solution  depends  on  the  amount  of  carbon  dioxide  present.  Thus 
in  regions  where  limestones  or  other  carbonate  rocks  abound,  the 
natural  waters,  under  atmospheric  pressures,  attack  such  rocks  and 
take  the  lime  carbonate  into  solution  in  a  relatively  slow  manner, 
but  in  spring  waters,  and  especially  thermal  ones  coming  from  depths, 
the  pressure  may  be  great,  the  amount  of  contained  carbon  dioxide 
large  and  the  quantity  of  dissolved  carbonate  of  lime  proportionately 
so.  Such  waters  on  coming  to  the  surface  lose  the  greater  part  of 
the  dissolved  carbonic  acid  in  the  form  of  gas,  and  the  lime  in  solu- 
tion is  consequently  deposited  rapidly  and  in  large  amount.  In  the 
waters  under  surface  atmospheric  pressure  the  lime  is  deposited 
by  evaporation  and  therefore  much  more  slowly.  In  warm  waters 
the  deposit  of  lime  may  be  much  increased  by  the  action  of  low 
forms  of  vegetable  life,  algae,  living  in  them,  which  secrete  lime  from 
the  water. 

The  rock  thus  formed  by  deposit  of  carbonate  of  lime 
from  solution  is  called  travertine,  from  the  old  Roman 
name  of  the  town  of  Tivoli  near  Rome,  where  an  exten- 
sive formation  of  the  substance  exists.  When  deposited 
slowly,  as  in  the  stalactites  and  stalagmites  in  caves,  it  is 
rather  hard  and  compact,  fine  crystalline,  sometimes 
white  but  usually  tinted  yellowish  or  brownish;  it  often 
has  a  fibrous  or  concentric  structure;  it  breaks  with  a 


PLATE  29. 


'  w^f*. 


A.   CALCAREOUS   TUFA,    DEPOSITED   ON   VEGETATION, 
COLORADO. 


B.   CALCAREOUS  TUFA,    YELLOWSTONE   PARK. 
(U.  S.  Geological  Survey.) 


DESCRIPTION  OF  STRATIFIED   ROCKS  313 

splintery  fracture.  When  deposited  more  rapidly,  as  by 
springs,  it  is  softer,  not  evidently  crystalline,  and  porous  to 
loose  or  earthy;  when  formed  coating  vegetation  it  may 
be  open,  cellular,  spongy,  bladed  or  moss-like,  as  illustrated 
in  Plate  29. 

These  looser,  less  compact,  varieties  are  commonly  called 
calcareous  tufa  or  calcareous  sinter.  Deposits  of  traver- 
tine, or  calcareous  tufa,  are  found  in  nearly  all  countries 
and  especially  in  limestone  regions.  Many  caves  are 
celebrated  for  the  number,  size  and  beauty  of  the  stalac- 
titic  and  stalagmitic  formations  they  contain.  See  Plate 
30.  Springs  depositing  carbonate  of  lime  are  very  com- 
mon, but  warm  carbonated  waters  are  chiefly  found  in 
volcanic  regions  or  those  which  have  recently  been  so, 
like  the  celebrated  Mammoth  Hot  Springs  of  the  Yellow- 
stone Park,  and  others  found  in  California,  Mexico,  Italy, 
New  Zealand,  etc.  See  Plate  31.  The  so-called  Mexican 
"  onyx  "  or  "  onyx  marble,"  which  is  extensively  used 
as  an  ornamental  stone,  is  a  travertine  with  a  banded 
structure,  beautifully  brought  out  by  its  varied  tinting 
through  metallic  oxides. 

Marl.  This  name  is  given  to  loose,  earthy  or  friable 
deposits  consisting  chiefly  of  intermingled  carbonate  of 
lime,  or  dolomite,  with  clay,  in  variable  proportions.  The 
color  is  usually  gray,  but  they  are  often  yellow,  green, 
blue  or  black,  and  sometimes  with  pronounced  color  tones 
due  to  some  special  substance,  as  oxide  of  iron  or  organic 
matter.  They  show  all  gradations  into  clays  and  shales. 
On  exposure  to  the  air  or  water  they  crumble  quickly 
into  coarse  soils.  The  carbonates  in  them  are  readily 
detected  by  their  effervescence  in  acid.  According  to 
special  substances  or  objects,  which  they  may  contain  in 
addition  to  those  mentioned,  different  varieties  are  named; 
thus  sandy  marl  is  full  of  grains  of  quartz  sand  and  often 
of  other  minerals;  shell  marl  is  a  whitish,  earthy  deposit 
formed  of  fragments  of  shells  of  various  organisms  formed 
in  enclosed  basins  of  water,  mingled  with  clay,  etc.  In 


314 


ROCKS  AND  ROCK  MINERALS 


the  Atlantic  and  Southern  states  this  name  is  applied  to 
beds  which  contain  abundantly  shells  of  mollusks,  gastro- 
pods and  other  shell-fish. 

The  chemical  composition  of  marls  varies  very  greatly; 
the  following  analysis  of  a  compact  one  from  Colorado  will 
serve  as  an  example. 


CaCO3 

MgC03 

Si02 

A12O3 

FezOa 

MgO 

K2O 

H2O 

Org. 
Sub. 

XyO 

Total 

21.6 

1.7 

45.9 

13.2 

3.9 

1.3 

2.3 

5.4 

3.5 

1.2 

100.0 

XyO  «=small  quantities  of  TiO2,Na£O  and  PZO8. 
Greensand  Marl  is  described  under  sandstones. 
PHOSPHORITE  —  PHOSPHATE  ROCKS. 

Deposits  of  phosphate  of  lime,  while  not  of  great  geo- 
logical importance  in  making  extensive  formations,  are 
yet  of  considerable  interest  and,  commercially,  of  great 
value,  from  their  use  as  soil  fertilizers.  When  occurring 
in  stratified  rocks  and  unconnected  with  igneous  intrusion 
they  represent  material  of  organic  origin.  While  some 
invertebrates,  such  as  a  few  species  of  brachiopods,  secrete 
phosphate  of  lime  in  their  shells  and  hard  parts,  it  is  mostly 
to  the  bones  and  excrement  of  vertebrates  that  the  origin, 
of  this  material  must  be  ascribed.  Sometimes  the  deposit 
appears  to  be  the  direct  and  original  one,  but  more  com- 
monly it  is  secondary  in  nature,  in  that  the  phosphates 
have  been  leached  out,  carried  down  and  redeposited  as 
nodular,  concretionary  or  lenticular  masses  in  clefts  and 
other  cavities  in  the  rocks  in  which  they  occur. 
Especially  in  limestones,  which,  being  soluble,  are  carried 
away,  the  less  soluble  phosphatic  material  tends  to  accu- 
mulate in  such  masses.  The  general  name  of  phosphate 
rock  or  phosphorite  may  be  used  for  all  such  material. 
The  appearance  of  these  rocks  is  variable,  sometimes  com- 


PLATE  30. 


STALACTITES   OF   TRAVERTINE    IN   LURAY   CAVERN, 

VIRGINIA. 
(U.  8.  Geological  Survey.) 


DESCRIPTION  OF  STRATIFIED  ROCKS 


315 


pact  semi-crystalline,  fibrous  or  concretionary,  often 
cavernous  or  spongy;  sometimes  in  rounded  mammillary 
forms;  in  other  cases  more  or  less  earthy.  The  color  is 
usually  gray,  but  sometimes  white,  buff,  reddish,  bluish, 
or  even  black.  The  simplest  test  for  these  phosphates  is 
to  dissolve  a  powdered  sample  in  nitric  acid,  and,  after 
filtering  off  the  insoluble  matter,  to  add  an  excess  of 
ammonium  molybdate  solution  and  ascertain  by  the 
yellow  precipitate  if  phosphorus  is  present.  The  general 
chemical  composition  is  shown  in  the  following  analyses 
of  samples  from  various  localities  in  North  Carolina. 


1.5 

31  2 

22.1 

23.4 

Carbonate  of  lime  

12.0 

15.9 

42.1 

64.3 

Phosphate  of  lime  

71.8 

42.1 

20.5 

11.2 

Water  and  other  constituents  

14.7 

10.8 

15.3 

1.1 

Total                       

100.0 

100.0 

100.0 

100.0 

Phosphorites  are  widely  distributed;  in  the  United 
States  they  are  found  extensively  in  the  Carolinas,  in 
Florida,  in  Tennessee,  and  in  some  of  the  other  states; 
they  occur  in  England  and  Wales,  in  Belgium,  northern 
France,  and  Russia. 

COAL  AND  OTHER  CARBONACEOUS  ROCKS. 

It  is  well  known  that,  interbedded  with  other  stratified 
rocks  of  the  different  geological  periods  down  to  the  present, 
there  occur  layers  of  carbonaceous  character,  which  under 
the  names  of  coal,  lignite,  etc.,  represent  the  remains  of 
former  vegetable  life,  which  once  flourished  where  these 
beds  are  now  found.  The  formation  of  peat  in  modern 
lakes,  swamps,  and  bogs,  and  its  occurrence  in  beds  inter- 
stratified  in  recent  delta  deposits  with  those  of  sands  and 
clays,  as  in  the  Mississippi  delta,  shows  us  how  these  beds 
of  coal  were  formed.  For  between  the  growing  vegetation 
of  to-day,  its  change  into  peat,  from  this  into  lignite  or 
brown  coal,  and  so  on  into  bituminous  coal,  then  into 


316  ROCKS  AND  ROCK  MINERALS 

anthracite,  and  eventually  into  graphite  or  practically 
pure  carbon,  every  step  of  gradual  transition  can  be 
traced. 

The  vegetable  matter  composing  plants  consists  for  the  most  part 
of  carbon,  hydrogen,  and  oxygen.  Its  decay  in  the  air,  like  combus- 
tion, is  a  process  of  oxidation;  the  hydrogen  goes  off  as  water,  the 
carbon  as  carbon  dioxide,  the  oxygen  of  the  air  assisting  that  in  the 
vegetable  matter  to  effect  the  change.  In  this  process  most  of  the 
carbon  is  removed.  If  the  decay  takes  place  under  water,  however, 
the  access  of  the  oxygen  of  the  air  is  prevented  and  the  process 
becomes  much  like  that  where  wood  is  burned  with  a  limited  amount 
of  air  to  form  charcoal.  Some  of  the  carbon  unites  with  some  of  the 
oxygen  to  form  carbon  dioxide;  some  of  the  hydrogen  unites  with 
the  rest  of  the  oxygen  to  form  water;  the  rest  of  the  hydrogen  unites 
with  some  of  the  carbon  to  form  marsh  gas  (methane)  and  the 
remainder  of  the  carbon  is  left  behind.  This  can  be  illustrated 
by  the  following  equation  in  which  the  formula  of  cellulose,  which 
comprises  the  most  important  part  of  vegetable  matter,  is  used. 

Cellulose    Carb.  Diox.  Water  Methane  Carbon 
s  -  COS  +  3H2O  +  CH4  +  4C. 


It  is  not  intended  to  imply  that  this  change  takes  place  at  once, 
or  is  complete,  under  water;  it  goes  on  gradually,  and  as  the  CO2 
and  CH«  are  evolved,  the  residual  matter  becomes  richer  in  carbon 
and  poorer  in  hydrogen  and  oxygen.  Thus  vegetable  matter  is 
converted  into  peat  and  this  by  compression  and  further  change 
into  brown  coal  or  lignite.  The  same  process  goes  on  in  coal  beds, 
furnishing  the  deadly  gases  known  to  the  miners  as  choke-damp 
(CO2)  and  fire-damp  (CH4),  and  lignite  thus  changes  to  bituminous 
coal.  Folding  of  the  strata,  with  compression  and  heat,  and  the 
consequent  rupturing  and  fissuring  of  the  overlying  beds,  which 
permits  the  easy  escape  of  the  gases,  hastens  the  process,  and  under 
such  circumstances  the  coal  is  changed  to  anthracite,  which  is  much 
richer  in  carbon,  or  even  into  graphitic  coal  which  is  practically 
pure  carbon.  Thus  the  degree  to  which  lignite  has  advanced  through 
bituminous  coal  to  anthracite  depends  in  part  on  its  geological  age, 
and  in  part  on  the  conditions  to  which  it  has  been  subjected.  . 

Peat.  This  varies  from  a  brown  to  yellowish  matted 
mass  of  interlaced  fibrous  material,  strongly  resembling 
compressed  tobacco,  in  which  remains  of  plant  leaves, 


PLATE   31. 


DESCRIPTION  OF  STRATIFIED  ROCKS 


317 


stems,  roots,  etc.,  are  still  recognizable,  in  the  upper  por- 
tion of  the  bed,  to  a  dark  brown,  or  black,  compact,  homo- 
geneous mass  appearing  much  like  dark  clay  when  wet, 
in  the  deeper,  lower  parts.  A  dried,  compact,  very  pure 
peat  from  Germany  is  stated  to  have  the  following 
composition: 


Carbon. 

Hydrogen. 

Oxygen. 

Nitrogen. 

Ash. 

Total. 

55.9 

5.8 

36.4 

1.0 

0.9 

100.0 

Under  enormous  pressure  it  has  been  found  that  peat  may  be 
artificially  converted  into  a  hard,  black  substance  like  coal.  The 
wide  distribution  of  peat  and  its  use  as  a  fuel  are  too  well  known  to 
need  further  mention.  Its  purity  depends  on  the  amount  of  clay 
and  sand  mingled  with  it  in  the  process  of  formation;  even  the  purest 
peat,  like  coal,  has  a  small  percentage  of  ash  resulting  from  the 
mineral  constituents  in  the  plants. 

Lignite.  Brown  Coal.  Usually  a  chocolate  brown  in 
color,  but  varying  to  yellowish  or  black;  compact  and 
firm  to  earthy  and  fragile;  luster  dull  and  soft  to  pitchy; 
often  shows  distinctly  the  texture  and  grain  of  wood  or 
intermatting  of  vegetable  fibers.  Hardness  varies  from 
1-2.5,  the  specific  gravity  from  0.7-1.5.  It  burns  readily 
with  a  smoky  yellow  flame,  and  strong  odor.  The  carbon 
in  it  varies  from  55  to  75  per  cent.  A  lignite  from  Ger- 
many is  stated  to  have  the  following  composition  which 
will  serve  as  an  illustration. 


Carbon. 

Hydrogen. 

Oxygen. 

Nitrogen. 

Ash. 

Total. 

57.1 

4.6 

36.0 

0.2 

2.0 

99.9 

Lignite,  belongs  in  the  Cretaceous  and  Tertiary  formations  and 
often  forms  considerable  beds  where  these  formations  occur.     It  is 


318 


ROCKS  AND   ROCK  MINERALS 


found  in  small  amount  in  the  eastern  United  States  in  the  Tertiary 
at  Brandon,  Vermont,  but  in  the  Cretaceous  deposits  of  the  Rocky 
Mountain  states,  and  in  the  Dakotas  it  occurs  in  large  and  valuable 
fields.  It  is  found  also  on  the  Pacific  Coast,  and  in  Germany  in 
Europe;  and  elsewhere.  Where  better  coal  is  not  to  be  had  it  fre- 
quently furnishes  a  valuable  fuel. 

Bituminous  or  Soft  Coal.  This  is  a  compact,  brittle 
rock  of  a  gray-black  to  velvet-black  color.  It  has  a 
lamellar,  conchoidal  or  splintery  fracture;  sometimes  more 
or  less  cubical.  The  luster  varies  from  dull  to  pitchy; 
the  specific  gravity  from  1.2-1.5.  It  gives  a  black  to 
brownish-black  streak.  It  burns  with  a  yellow  flame  and 
gives  a  strong  bituminous  odor.  It  often  shows  distinct 
stratification  through  the  varying  luster  of  the  different 
layers.  Generally  there  are  no  traces  of  organic  struc- 
tures visible  to  the  eye.  Some  varieties  fuse  or  sinter 
together  on  heating,  leaving  a  coherent  residue  or  coke, 
and  are  thus  called  coking  coals;  others  fail  to  do  this  and 
fall  to  powder.  The  amount  of  carbon  in  a  soft  coal 
varies  from  75-90  per  cent.  A  coking  coal  from  Northum- 
berland in  England  has  been  found  to  have  the  following 
composition. 


Carbon. 

Hydrogen. 

Oxygen. 

Nitrogen. 

Sulphur. 

Ash. 

Total. 

78.7 

6.0 

10.1 

2.4 

1.5 

1.4 

100.1 

The  sulphur  in  coal  comes  from  pyrite,  which  is  a  very  common 
impurity.  Bituminous  coals  vary  considerably  in  the  relative 
proportions  of  fixed  carbon  to  volatile  matter,  that  is  in  the  pro- 
portion of  the  carbon  left  behind  on  heating  to  the  gases,  tar,  etc., 
driven  off;  the  latter  may  be  as  much  as  30-40  per  cent  and  such 
coals  are  called  fat  coals  and  are  used  for  the  making  of  gas,  coke, 
etc.  Those  with  15-20  per  cent  volatile  matter  are  largely  used  for 
steam  engines  and  are  often  called  steam  coals.  They  are  transi- 
tional to  anthracite. 

In  addition  to  ordinary  coal  there  are,  depending  on  the  physical 


DESCRIPTION  OF    STRATIFIED  ROCKS  319 

characters,  a  number  of  varieties  which  are  well  recognized.  Thus 
cannel  coal  is  a  dense,  lusterless,  highly  bituminous  form  without 
structure  and  generally  showing  conchoidal  fracture.  Jet  is  some- 
what similar  but  characterized  by  its  high  luster,  intense  black  color, 
asphaltic  appearance,  and  toughness,  which  permits  of  its  being 
readily  turned  and  worked.  Its  use  in  mourning  jewelry,  buttons, 
ornaments,  etc.,  is  well  known.  It  occurs  in  small,  scattered,  isolated 
masses  in  the  later  formations  in  various  places,  one  of  the  chief 
localities  being  at  Whitby  in  Yorkshire,  England.  It  is  regarded 
by  some  as  representing  water-logged  fragments  of  originally 
coniferous  wood.  Bituminous  coal  occurs  in  North  America  in 
Nova  Scotia;  in  the  Appalachian  coal  field  of  western  Pennsylvania, 
Ohio,  West  Virginia,  Kentucky,  Tennessee,  Alabama  and  Georgia ;  the 
Central  coal  field  of  Illinois,  Indiana  and  Kentucky;  in  Michigan;  the 
Western  field  of  Iowa,  Missouri,  Kansas,  Arkansas,  Oklahoma  and 
Texas.  These  are  of  Carboniferous  age.  In  the  Rocky  Mountain 
states  and  on  the  Pacific  coast  there  are  also  large  deposits  of  Creta- 
ceous and  Tertiary  age.  Elsewhere,  in  England,  Belgium,  Germany, 
France  and  Russia,  in  South  Africa,  Australia,  India  and  China,  this 
coal  occurs  and  is  mined  in  quantities.  It  is  the  chief  coal  of  the 
world,  and  the  enormous  increase  in  production  in  these  later  years 
(in  the  United  States  from  137,000,000  tons  in  1896  to  463,000,000 
tons  in  1922)  points  in  no  uncertain  way  to  its  exhaustion  in  a  not 
distant  future. 

Anthracite.  Hard  Coal.  This  is  a  compact,  dense 
rock,  iron-black  to  velvet-black  in  color.  It  is  brittle; 
has  a  strong  vitreous  to  sub-metallic  luster,  a  more  or  less 
pronounced  conchoidal  fracture,  and  a  hardness  of  2-2.5. 
Specific  gravity  1.4-1.8.  Anthracites  vary  in  the  amount 
of  carbon  they  contain  from  80-95  per  cent;  Pennsylvania 
varieties  from  85-93  and  of  Wales  88-95.  The  amount  of 
fixed  carbon  varies  from  80-90  per  cent;  the  volatile  hydro- 
carbons generally  do  not  much  exceed  5  per  cent  and  the 
remainder  consists  of  moisture  and  ash.  An  analysis  of 
a  Welsh  anthracite  is  given  as: 

Carbon    Hydrogen    Oxygen    Nitrogen    Sulphur    Ash    Total 
90.4  3.3  3.0  0.8  0.9         1.6  =  100.0 

Anthracite  requires  a  strong  heat  for  ignition  and  with  abundant 
access  of  air  burns  with  a  pale  blue  flame,  giving  great  heat  without 
smoke  or  odor.  These  qualities,  with  its  relative  cleanliness,  par- 


320  ROCKS  AND  ROCK  MINERALS 

ticularly  adapt  it  to  household  purposes.  Some  anthracites  exhibit 
on  broken  surfaces  a  strong  play  of  spectrum  colors  produced  by 
iridescent  films  and  are  called  "  peacock  "  coal. 

Anthracite  occurs  not  only  in  regions  of  folded  strata  as  previously 
stated,  but  also,  though  usually  in  no  great  quantity,  where  beds 
containing  ordinary  bituminous  coal  or  lignite  have  been  invaded 
by  intrusive  masses  of  igneous  rock,  as  in  New  Mexico,  Colorado, 
Montana  and  Scotland.  The  largest  and  most  important  deposits  of 
anthracite  are  those  of  eastern  Pennsylvania,  a  considerable  part 
of  which  has  been  already  mined.  It  occurs  also  in  Wales,  Belgium, 
France,  Russia,  and  in  the  province  of  Shansi  in  China,  as  well  as  in 
other  places. 

In  addition  to  the  carbonaceous  rocks  described  above, 
other  carbonaceous  and  sometimes  combustible  substances 
occur,  such  as  graphite,  ozokerite  or  mineral  wax,  asphalt 
and  various  modifications  of  it,  petroleum,  etc.,  but  not  in 
such  a  manner  or  relation  that  they  may  be  properly  in- 
cluded in  a  work  treating  solely  of  rocks.  Also  between 
the  coals  and  the  sandstones  and  shales,  intermediate 
types  exist  in  great  variety,  but  these  are  best  treated 
under  the  description  of  the  latter  rocks. 

Sedimentary  Deposits  of  Mechanical  Origin. 

These  include  the  products  of  land  waste  by  various 
erosive  agencies,  which  have  been  laid  down  in  stratified 
form,  by  moving  currents  of  water  in  seas,  lakes,  and  on 
the  flood-plains  of  rivers,  and  afterwards  consolidated 
into  rocks,  as  described  in  the  foregoing  chapter.  Accord- 
ing to  the  size- of  the  particles  they  are  divided  into  the 
gravel  rocks  or  conglomerates  and  breccias,  into  the  sand 
rocks  or  sandstones,  and  into  the  mud  or  clay  rocks,  or 
shales,  as  previously  mentioned. 

CONGLOMERATES   AND   BRECCIAS. 

Conglomerates.  These  consist  of  pebbles  of  various 
sizes,  intermingled  with  a  finer  material  which  acts  as  a 
cement.  The  pebbles  may  vary  from  the  size  of  a  pea  up 
to  large  boulders.  They  are  rounded  by  the  action  of 


PLATE   32 


A.   CONGLOMERATE,    OF   SEDIMENTARY   ORIGIN. 


B.   BRECCIA,    OF   SEDIMENTARY   ORIGIN. 


DESCRIPTION  OF  STRATIFIED   ROCKS  321 

water.  They  may  consist  of  any  kind  of  rock,  though 
generally  of  the  harder  and  more  resistant  varieties,  or 
they  may  be  of  a  simple  mineral,  usually  quartz  or  feldspar. 
The  pebbles  may  be  all  of  one  kind  or  of  a  mixture  of 
several  kinds  of  rocks  or  minerals.  The  cementing  mate- 
rial may  also  vary  greatly;  it  may  be  composed  chiefly  of 
consolidated  sand,  either  purely  siliceous  or  mixed  sub- 
stances; it  may  be  calcareous  in  nature,  or  chiefly  com- 
posed of  clay,  or  of  these  substances  largely  mingled  with 
iron  oxide.  There  may  be  a  sharp  distinction  between 
the  relatively  large  pebbles,  and  the  very  fine  matrix  in 
which  they  are  enclosed,  and  if  this  contrast  is  pronounced 
and  the  matrix  present  in  considerable  amount,  such 
conglomerates  are  often  called  pudding  stone.  On  the 
other  hand  there  may  be  gradations  in  size  from  the  peb- 
bles down  into  the  matrix.  There  is  of  course  great  vari- 
ation in  the  color  of  these  rocks ;  in  some  cases  the  pebbles 
are  sharply  defined  by  their  colors  from  the  matrix;  in 
other  cases  the  rock  may  have  one  general  hue,  alike  for 
pebbles  and  matrix  —  this  is  more  apt  to  be  the  case 
where  the  rock  has  been  somewhat  changed  or  altered 
from  its  original  character. 

Breccias.  In  a  breccia  the  fragments  which  correspond 
to  the  pebbles  of  a  conglomerate,  instead  of  being  rounded, 
are  sharp  and  angular  in  character.  (See  Plate  32.) 
This  indicates,  if  the  material  has  been  laid  down  in 
water,  that  they  have  suffered  very  little  transport  and 
are  close  to  their  place  of  origin.  In  other  respects  what 
has  been  said  in  regard  to  conglomerates  will  also  apply 
to  breccias. 


Conglomerates  and  breccias,  which  are  composed  of  a  single  type 
of  rock,  are  generally  called  by  its  name  and  we  thus  have  quartz- 
ite  conglomerates  and  breccias,  limestone  conglomerates  and 
breccias,  etc.  Volcanic  breccias,  produced  by  the  fragmental 
accumulations  of  eruptive  activity,  are  really  igneous  rocks  and 
have  been  already  described  (see  page  269).  The  material  may, 
however,  fall  into  water  and  be  rounded,  assorted,  and  stratified, 


322  ROCKS  AND  ROCK  MINERALS 

giving  rise  to  volcanic  conglomerates;  such  rocks  are  very  difficult,  and 
sometimes  impossible,  to  distinguish  from  conglomerates  formed 
by  the  erosion  of  such  land  areas  as  are  formed  for  the  most  part  of 
surface  extrusions  of  lavas. 

Breccias  are  sometimes  produced  as  the  result  of  the  breakage 
and  grinding  of  the  rock  masses  along  some  fault  plane  upon  which 
powerful  movement  is  occurring.  The  fragments  thus  formed  may 
be  afterwards  cemented  together  into  firm  rock  by  deposits  from 
solutions  circulating  in  the  zone  of  crushed  and  broken  rock.  Such 
types  are  called  friction  breccias  and  they  naturally  show  no  evi- 
dence of  stratification. 

Conglomerates  are  normally  formed  from  deposits  laid  down  by 
swiftly  moving  currents  of  water  which  tend  to  carry  away  the 
lighter  and  finer  material  in  suspension.  Hence  they  represent  the 
deposits  of  rapid  rivers  and  estuarine  currents.  Also,  when  a  sink- 
ing land  surface  passes  under  the  sea  and  the  edge  of  the  latter 
advances,  a  beach  formation  sweeps  over  the  land  as  the  initial 
stage  to  later  deposits.  The  waves  throw  the  coarser  material, 
the  gravel  or  shingle,  toward  the  upper  part  of  the  beach  and  as  the 
latter  sweeps  inland  a  conglomerate  is  thus  the  first  deposit  laid 
down  on  the  new  sea  bottom.  Thus  it  is  general  to  find  a  conglom- 
erate or  coarse  sandstone  as  the  first  member  of  a  new  series  of 
stratified  rocks,  resting  upon  an  unconformable  lower  series,  and  in 
thus  marking  divisions  of  geologic  time  they  may  be  of  great  im- 
portance. They  are  quite  common  rocks  and  are  everywhere  dis- 
tributed in  the  sedimentary  formations. 

In  the  older,  and  especially  in  strongly  folded  mountain  regions 
where  the  strata  have  suffered  great  pressures  and  shearing,  the 
pebbles  of  conglomerates  are  generally  distorted  and  flattened  into 
lenses,  or  drawn  out  into  spindle-shaped  forms.  The  process  is 
generally  accompanied  by  mineralogical  changes  which  may  be 
especially  noticeable  in  the  cement.  This  is  the  first  stage  in  the 
conversion  of  these  rocks  into  gneisses  and  schists  through  meta- 
morphism,  as  described  in  the  following  chapter.  On  account  of 
their  coarse  and  irregular  appearance  and  unhomogeneous  char- 
acter conglomerates  have  been  little  used  for  structural  purposes, 
except  in  the  roughest  stone  work,  as  in  foundations,  piers,  etc.  In 
some  cases  breccias,  which  are  compact  and  capable  of  a  good 
polish,  have  been  cut  as  ornamental  stones,  as  a  reddish  conglomerate 
breccia  from  South  Dakota  and  a  vari-colored  limestone  breccia 
from  Japan.  Since  the  discovery  of  the  wonderful  gold  deposits 
in  conglomerates  in  the  Rand  district,  South  Africa,  these  rocks 
have  received  much  attention,  as  representing  possible  fossil  placers 
in  which,  if  the  gold  has  been  concentrated  by  natural  processes, 
available  sources  of  the  precious  metal  might  be  expected. 


DESCRIPTION   OF  STRATIFIED  ROCKS  323 


SANDSTONE   AND   RELATED   ROCKS. 

Sandstone.  Typical  sandstones  are  composed  of  grains 
of  quartz  held  together  by  some  substance  acting  as  a 
cement.  The  size  of  grain  may  vary  from  that  of  peas 
down  to  that  of  fine  seeds;  as  they  become  finer  the  rocks 
pass  into  shales,  just  as  on  the  other  hand  they  graduate 
upward  into  conglomerates,  and  thus  no  sharp  line  can 
be  drawn  between  the  three  kinds.  While  some  sand- 
stones are  very  pure,  consisting  of  quartz  grains  alone, 
others  contain  intermingled  particles  of  feldspar,  garnet, 
iron  ore,  tourmaline,  flakes  of  mica  and  fragments  of  other 
minerals.  It  can  generally  be  observed  with  a  lens  that 
the  grains  tend  to  be  spheroidal,  and  that  the  larger  they 
are,  the  more  perfect  the  rounding  is  apt  to  be.  The 
general  appearance  of  many  sandstones,  with  respect  to 
their  granular  texture,  is  much  like  that  of  loaf  sugar. 
As  described  under  quartzite,  to  which  reference  should  be 
made,  the  fracture,  in  breaking  sandstone,  takes  place 
chiefly  in  the  cement,  leaving  the  grains  outstanding,  and 
this  gives  the  rock  its  sugary  appearance  and  feeling. 

Sandstones  differ  very  much  in  regard  to  the  cementing 
material  which  holds  the  grains  together,  and  thus  different 
varieties  are  produced.  Sometimes  it  is  deposited  silica, 
sometimes  a  carbonate  —  commonly  calcite,  but  on  occa- 
sion dolomite  or  siderite,  —  sometimes  extremely  fine 
argillaceous  material  or  clay,  and  at  other  times  deposited 
oxides  of  iron,  either  reddish  (hematite,  turgite),  or  yellow- 
ish (limonite). 

The  colors  are  very  variable,  white  to  gray,  buff  to 
dark  yellow,  and  brick-red  to  reddish  bro.wn  and  brown,  are 
common;  green,  purple  and  black  are  rare.  These  colors 
depend  largely  on  the  nature  of  the  cement;  in  the  yellow, 
red  and  brown  sandstones  oxides  of  iron  predominate,  with 
the  other,  lighter  colors,  it  is  apt  to  be  calcareous  or  argilla- 
ceous. In  addition,  the  calcareous  sandstones  are  readily 
detected  by  their  effervescing  when  touched  with  acid, 


324 


ROCKS  AND  ROCK  MINERALS 


while  the  argillaceous  ones  give  the  characteristic  odor  of 
clay,  when  breathed  upon.  The  green  color  is  due  to 
glauconite,  or  in  some  cases  admixed  chlorite.  Some 
varieties  appear  to  be  almost  devoid  of  a  cement. 

Sandstones  are  usually  very  porous  rocks,  and  this 
appears  to  depend  to  a  large  extent  upon  the  amount  and 
character  of  the  interstitial  cement.  Thus  the  ratio  of 
the  volume  of  pore  space  to  that  of  the  rock  has  been 
found  to  vary  from  5  to  almost  30  per  cent,  the  latter 
being  about  the  greatest  amount  theoretically  possible  in 
deposited  sand  grains. 

The  same  characters  also  condition  to  a  large  degree 
other  physical  properties  and  also  explain  their  variations : 
thus  the  weight  per  cubic  foot  varies  from  125-150  pounds, 
the  crushing  strength  from  1500  to  15,000  pounds  per 
square  inch.  The  specific  gravity  is  about  2.6  (2.5-2.7), 
with  the  rock  pores  filled  with  water,  when  weighed  in  it. 

The  chemical  composition  of  sandstone  varies  con- 
siderably; the  chief  element  is  silica,  but  the  proportions 
of  the  other  elements  depend  on  the  nature  of  the  asso- 
ciated minerals  and  cement.  Some  analyses  of  promi- 
nent sandstones  used  for  building  purposes  are  as  follows: 


SiO2 

A12O3 

Fe2O3 

FeO 

MgO 

CaO 

K2O 

Na^O 

H2O 

CO2, 
etc. 

Total 

I.... 

99.4 

0.3 

0.2 

99.9 

II... 

86.6 

8.4 

1.6 

— 

— 

trace 

2.4 

0.7 

_ 

_ 

99.7 

III.. 

92.9 

3.8 

trace 

0.9 

trace 

0.3 

0.6 

0.3 

1.2 

_ 

100.0 

IV  .. 

69.9 

13.6 

2.5 

0.7 

trace 

3.1 

3.3 

5.4 

1.0 

_ 

99.5 

V  ... 

87.1 

3.9 

1.3 

_ 

1.1 

2.7 

1.3 

0.8 

0.5 

1.4 

100.1 

VI  .. 

90.7 

4.6 

0.4 

0.1 

0.1 

0.1 

0.5 

2.8 

0.4 

0.3 

100.0 

I,  White,  very  pure  Potsdam  Sandstone,  Ablemans,  Sauk  Co., 
Wisconsin;  II,  Lake  Superior  Brownstone,  Houghton,  Bayfield  Co., 
Wisconsin;  III,  Sandstone,  light  gray,  Berea,  Ohio;  IV,  Brownstone, 
Triassic,  Portland,  Conn.;  V,  Sandstone,  Triassic,  near  Liverpool, 
England;  VI,  Bunter  sandstone,  Heidelberg,  Germany. 


PLATE  33. 


A.    SANDSTONE,    OF    FINE   GRAIN. 


B.    LAMINATED   SANDSTONE,    WITH   SLIGHT    FAULTS. 


DESCRIPTION   OF   STRATIFIED  ROCKS 


325 


The  presence  of  the  alkalies  points  to  that  of  feldspar 
(or  mica)  in  the  rock;  in  IV  the  amount  of  feldspar  must 
be  large,  and  such  a  rock  is  to  be  classed  as  an  arkose 
rather  than  a  sandstone. 

The  structure  of  sandstones  is  essentially  that  of  the  stratified 
rocks.  They  are  sometimes  thinly  laminated  and  fissile,  and  some- 
times very  thick  bedded  and  within  the  individual  bed  may  show 
a  very  even  texture  and  be  practically  free  from  any  evidence  of 
stratification.  Sandstones  such  as  the  latter  are  valuable  for 
structural  purposes  on  account  of  their  homogeneous  character 
and  capability  for  cleaving  or  working  equally  in  all  directions; 
they  are  often  called  freestones. 

These  rocks  are  frequently  distinguished  according  to  the 
character  of  the  cement  or  admixed  material  as  described  above; 
thus  there  are  calcareous  sandstones,  argillaceous  sandstones, 
jerrugineous  sandstones  and  siliceous  sandstones.  Micaceous  sand- 
stones contain  considerable  muscovite;  the  tabular  flakes  are  par- 
allel to  the  bedding  and  induce  a  more  or  less  ready  cleavage  in  the 
rock,  giving  it  a  fissile  character;  the  cleavage  faces  are  apt  to  be 
somewhat  silvery  in  appearance  from  the  mica  films  coating  them. 
Grit  is  a  term  applied  to  coarse-grained  sandstones  whose  particles 
are  in  general  more  or  less  sharply  angular,  and  whose  cementing 
material  is,  as  a  rule,  quite  siliceous.  They  have  been  considerably 
used  for  grindstones  and  millstones,  hence  the  term  "  millstone  grit." 
In  siliceous  sandstones  it  may  happen  that  the  deposited  silica  is 
precipitated  upon  the  rounded  or  angular  quartz  grains  in  crys- 
talline position,  thus  reconverting  them  outwardly  into  crystals; 
examination  with  the  lens  shows  the  crystal  forms  and  faces  of  the 
little  regenerated  quartzes;  these  are  known  as  crystal  sandstones. 

Green  sandstone  is  a  variety  full  of  grains  of  glauconite  which 
impart  a  general  greenish  color  to  the  rock.  Sometimes  these 
rocks  are  very  friable,  indeed  scarcely  coherent,  as  in  the  Cretaceous 
formations  of  the  Atlantic  border,  especially  in  New  Jersey.  They 
are  then  called  greensand  or,  inappropriately,  greensand-marl.  They 
are  apt  to  contain,  in  addition  to  the  sand  and  glauconite,  iron  oxides 
and  fossil  shells,  either  whole  or  fragmentary.  These  deposits  have 
been  considerably  used  as  fertilizers.  Analyses  of  typical  green- 
sands  from  New  Jersey  are  as  follows: 


SiO2 

P205 

SO3 

A1203 

Fe2O3FeO 

MgO 

CaO 

K2O 

HjO 

Total 

34.5 
51.2 

1.2 
0.2 

1.3 
0.4 

6.0 
8.2 

31.5 
23.1 

2.2 
2.0 

2.5 
0.5 

1.5 

7.1 

18.8 
6.7 

99.5 
99.4 

326        ROCKS  AND  ROCK  MINERALS 

Arkose.  This  is  a  special  variety  of  sandstone  in  which  a  notable 
quantity  of  feldspar  grains  is  mingled  with  those  of  quartz.  Often 
there  is  considerable  mica  present  and,  if  the  material  is  firmly 
cemented,  the  rock  to  a  casual  glance  may  bear  no  small  resemblance 
to  a  granite.  The  particles  are  generally  sharply  angular,  and  the 
feldspar  is  apt  to  be  soft  and  more  or  less  changed  to  kaolin.  Under 
a  lens  the  irregular,  clastic,  angular  shape  of  the  particles  readily  dis- 
tinguishes it  from  a  granite.  The  mineral  composition  and  the  shape 
of  the  grains  show  that  the  material  has  been  derived  from  quickly 
disintegrating  granite  and  has  suffered  but  a  very  short  transport 
before  being  deposited.  Arkoses  often  grade  into  conglomerates 
and  breccias  by  increasing  size  of  some  of  the  particles.  They 
occur  in  all  of  the  different  geological  formations.  The  red-brown 
Triassic  sandstones  of  New  England  are  in  large  part  arkose  and 
conglomerate  or  breccia. 

Graywacke.  These  are  sandstone-like  rocks  of  a  prevailing  gray 
color,  sometimes  brown  to  blackish,  which,  in  addition  to  the  quartz 
and  feldspar  of  an  arkose,  contain  rounded  or  angular  bits  of  other 
rocks,  such  as  fragments  of  shale,  slate,  quartzite,  granite,  felsite, 
basalt,  etc.,  or  of  varied  minerals,  hornblende,  garnet,  tourmaline, 
etc.  They  are  in  reality  fine-grained  conglomerates  and  readily 
pass  into  them  by  increase  in  size  of  some  of  the  component  particles. 
The  amount  of  cement,  as  in  sandstones,  is  usually  small  and  it  is 
generally  argillaceous,  but  sometimes  siliceous  or  calcareous.  Such 
rocks,  when  fine  grained  and  compact  and  largely  composed  of  feld- 
spathic  material,  may  be  difficult  in  the  hand  specimen  to  distinguish 
from  some  felsites,  but  close  examination  with  a  good  lens  will 
generally  show  their  nonhomogeneous  character.  The  name  has 
been  rather  loosely  used  and  has  never  had  the  vogue  in  America 
that  it  has  in  Europe. 

Uses  of  Sandstone.  As  is  well  known,  sandstone  is  everywhere 
used  for  constructional  purposes.  The  ease  with  which  it  is  worked, 
and  the  large  size  of  the  blocks  which  may  be  quarried,  make  it 
particularly  valuable  for  this  purpose.  Thus  in  the  United  States 
a  very  considerable  portion  of  the  buildings  of  the  eastern  cities  are 
wholly  or  in  part  of  the  red-brown  sandstone,  generally  called 
"  brownstone,"  of  the  Triassic  areas  of  the  Atlantic  border,  while 
for  instance  in  Great  Britain  the  city  of  Edinburgh  is  largely  built 
of  the  Carboniferous  sandstones  of  that  region.  On  account  of  the 
insoluble  nature  of  the  iron  oxide  forming  their  cement,  the  red  and 
brown  sandstones  in  moist  climates  retain  much  better  the  details  of 
fine  cutting  and  carving  for  architectural  effects,  than  do  the 
lighter  colored  gray  or  buff  stones.  The  latter  are  liable  to  have  a 
calcareous  cement,  which  dissolves  under  the  action  of  atmospheric 
agencies  and  water,  allowing  the  stone  to  crumble,  and  thus  in  the 


DESCRIPTION  OF   STRATIFIED  ROCKS  327 

course  of  years  the  fine  details  of  carving  are  spoiled.  Many  ex- 
amples of  this  may  be  seen  in  the  older  cities  where  expensive  and 
beautiful  buildings  have  been  much  injured.  If  possible,  in  building, 
a  sandstone  should  always  be  laid  upon  the  quarry  bed  as  it  is  then 
much  less  liable  to  flake  or  spall. 

Sandstones  are  of  such  wide  and  general  distribution 
in  all  parts  of  the  world  where  stratified  rocks  are  found, 
that  it  is  unnecessary  to  give  any  detailed  account  of  their 
occurrence. 

SHALE  AND  BELATED  ROCKS. 

Shale  is  the  name  given  to  compacted  muds  and  clays 
which  possess  a  more  or  less  thinly  laminated,  or  fissile 
structure.  The  parting  is  parallel  to  the  bedding,  and  is 
the  result  of  natural  stratification.  When  such  rocks 
have  been  subjected  to  folding  and  pressure,  they  assume 
a  slaty  cleavage  which  has  nothing  to  do  with  stratifica- 
tion; they  are  then  slates  or  phyllites  and  are  described 
among  the  metamorphic  rocks.  This  distinction,  that 
rocks  showing  slaty  cleavage  are  not  shales,  should  be 
clearly  noted,  as  the  two  are  often  confused. 

Shales  are,  in  general,  too  fine  grained  for  the  component 
particles  to  be  determined  with  the  eye,  or  even  with  the 
lens.  By  microscopical  and  chemical  analysis  they  are 
known  to  be  formed  mostly  of  kaolin  and  related  sub- 
stances, with  which  may  be  associated  much  white  mica, 
but  these  are  often  accompanied  by  tiny  fragments  of 
quartz  and  other  minerals.  As  the  amount  of  quartz 
increases,  and  also  the  size  of  grain,  the  shales  pass  over 
into  sandstones,  and  such  intermediate  rocks  represent 
deposited  silts.  There  are  also  all  transitions  between 
clays  and  shales,  depending  on  the  relative  firmness  and 
fissility  of  the  mass. 

Clay  when  dry  is  a  fine,  earthy,  lusterless  mass,  giving  a  char- 
acteristic odor  when  breathed  upon.  It  clings  to  the  tongue,  and 
when  strongly  rubbed  to  a  powder  between  the  fingers,  it  finally 
produces  a  soft,  greasy,  lubricated  feeling,  usually  thus  differing 
from  loess,  adobe,  and  similar  appearing  deposits.  It  absorbs  water 


328 


ROCKS  AND   ROCK  MINERALS 


eagerly  and  becomes  plastic.  When  pure  it  is  white,  but  it  is 
generally  colored  red  or  yellow  by  iron  oxides,  forming  the  red  and 
yellow  ochers,  or  gray,  blue  or  black  by  organic  substance.  The 
colors  are  sometimes  evenly  distributed,  and  sometimes  irregularly 
blotched,  through  the  mass. 

Shales  are  apt  to  be  soft,  cut  more  or  less  readily  with 
the  knife,  and  are  brittle  and  crumbly,  so  that  taken  in 
connection  with  the  fissility,  it  is  often  difficult  to  prepare 
hand  specimens  of  them.  Like  clays  they  exhibit  a  great 
variety  of  colors,  white  to  buff  or  yellow,  red  to  brown, 
purple,  greenish  and  gray  to  black,  and  from  the  same 
causes.  Different  shades  of  gray  are  perhaps  the  most 
common.  They  often  contain  various  accessory  mineral 
substances,  such  as  carbonates,  gypsum,  rock-salt,  pyrite, 
etc.  Some  of  these  are  frequently  seen  in  the  form  of 
concretions,  which  may  attain  large  size,  up  to  several  feet 
in  diameter. 

The  chemical  composition  is  somewhat  variable,  depend- 
ing on  the  relative  proportions  of  clay  and  other  minerals. 
The  following  analyses  will  serve  to  show  the  general 
chemical  character. 


SiO2 

A12O3 

Fe2O3 

FeO 

MgO 

CaO 

NaaO 

K2O 

H2O 

XyO 

Total 

55.0 

21.0 

5.0 

1.5 

2.3 

1.6 

0.8 

3.2 

8.1 

1.9 

100.4 

60.6 

16.4 

4.9 

_ 

1.4 

1.6 

0.9 

3.0 

9.7 

1.5 

100.0 

61.2 

15.6 

1.4 

3.0 

4.2 

3.4 

0.4 

6.7 

2.7 

1.1 

99.7 

53.6 

17.6 

4.1 

3.7 

5.2 

2.3 

2.5 

2.2 

8.5 

0.2 

99.9 

I,  Cambrian  Shale,  Coosa  Valley,  Cherokee  Co.,  Alabama; 
TI,  Cretaceous  Shale,  near  Pueblo,  Colorado;  III,  Devonian  Shale, 
Morenci  district,  Arizona;  IV,  Cretaceous  Shale,  Mount  Diablo. 
California. 

XyO  =  Carbonaceous  matter,  CO2,  and  small  amounts  of  other 
substances. 

There  are  many  varieties  of  shales,  depending  chiefly  on  the 
presence  of  accessory  materials.  Thus  there  may  be  a  large  amount 


DESCRIPTION   OF  STRATIFIED  ROCKS  329 

of  organic  matter,  mostly  carbon,  present,,  and  such  are  called  car- 
bonaceous sfiales.  They  are  black  in  color,  and  by  increase  of  car- 
bon, grade  into  coaly  shales,  shaly  coals  and  so  on  into  coal.  They 
are  a  very  common  type,  and  are  found  associated  with  coal  and  also 
independently  of  it,  sometimes  covering  wide  areas  and  of  great 
thickness.  From  the  nature  of  the  organic  matter  they  are  some- 
times called  bituminous  shales.  It  is  probable  that  the  total  amount 
of  carbon  in  the  shales  far  exceeds  that  existing  in  coal  beds. 

In  other  varieties  of  shales  large  amounts  of  carbonates,  especially 
carbonate  of  lime,  are  present,  and  these  are  known  as  calcareous 
shales.  By  increase  of  this  substance  they  pass  into  shaly  lime- 
stones. They  are  apt  to  be  associated  with  limestones  and  these 
calcareous  varieties  are  detected  by  their  ready  effervescence  with 
acids.  Or  the  carbonate  present  may  be  chiefly  carbonate  of  iron 
and  thus  produce  transition  forms  between  shales  and  clay  iron- 
stone previously  described.  The  connection  between  clays,  shales 
and  marls  has  been  mentioned  on  a  previous  page.  Alum  shale  is 
a  variety  full  of  pyrite,  or  of  sulphates  resulting  from  its  alteration; 
it  has  been  used  for  the  manufacture  of  alum. 

Uses  of  Clay  and  Shale.  The  use  of  clay  in  the  making 
of  bricks,  tiles,  pottery,  etc.,  is  too  well  known  to  need 
further  comment.  Shale  has  no  value  for 'structural  pur- 
poses, but  in  recent  years,  along  with  clay,  it  has  become 
of  value  and  is  used  in  many  places  as  a  material  for  the 
manufacture  of  Portland  cement,  when  mixed  with  the 
proper  proportion  of  limestone  and  burned.  A  pure, 
clean  shale  or  clay  of  the  general  composition  shown  in 
analysis  No.  II,  given  above,  is  one  best  adapted  for  this 
purpose,  when  combined  with  a  non-magnesian  limestone. 

Clays  and  shales  are  such  common  rocks  in  all  parts  of 
the  world,  where  the  unmetamorphosed  stratified  forma- 
tions are  found,  that  their  occurrence  needs  no  special 
description. 

Surficial  Deposits. 

This  small  group  of  geologic  materials  is  of  somewhat 
diverse  origin,  and  they  are  here  included  under  this  head- 
ing largely  as  a  matter  of  convenience.  They  would 
include  seolian  deposits,  or  those  made  by  the  wind,  and 
those  formed  by  the  disintegration  and  decay  of  previously 


830  ROCKS  AND  ROCK  MINERALS 

existent  rocks.  They  are  appended  here  to  the  stratified 
rocks,  because  they  are  in  general  closely  connected  with 
them,  and  in  many  cases  pass  insensibly  into  them. 
Many  indeed,  which  might  be  classed  here  wholly  or  in 
part,  have  already  been  described  elsewhere  because  of 
their  close  connection  with  other  rocks.  Thus  volcanic 
tuffs  and  breccias  have  been  described  under  igneous  rocks; 
clays  under  shale,  marl  under  carbonate  rocks,  sands  in 
Chapter  VIII,  etc.  Most  of  these  substances,  under  the 
ordinary  usage  of  the  word,  would  not  be  considered  rocks 
at  all,  and  their  treatment  entails  matters  of  geological 
interest  rather  than  such  as  enter  into  a  work  of  this  char- 
acter. This  applies  to  such  things,  for  instance,  as  soils, 
talus  heapings,  morainal  deposits  from  glaciers,  etc.  Their 
description  and  mode  of  origin  should  be  sought  in  the 
handbooks  on  geology,  or  in  special  manuals.  Only  a 
few  of  them,  which  from  their  widespread  occurrence  and 
great  importance  as  geological  formations  are  of  particulai 
interest,  are  here  included. 

Loess.  This  is  a  deposit  of  a  pale  to  buff  yellow  color, 
running  into  brown,  of  an  exceedingly  fine  grain;  friable, 
with  scarcely  the  consistency  of  ordinary  chalk  when 
coherent,  and  passing  into  looser  forms,  and  of  a  rather 
harsh  feeling  when  rubbed  between  the  fingers.  It  is  of 
a  remarkably  homogeneous  appearance,  and  commonly 
shows  no  signs  of  stratification,  though  this  is  sometimes 
clearly  seen.  It  has  been  found  to  consist  chiefly  of 
angular  grains  of  quartz,  mixed  with  considerable  amounts 
of  clay-like  substances,  tiny  specks  of  other  minerals,  and 
a  calcareous  cement,  the  amount  of  carbonate  of  lime 
rising  in  some  cases  to  30  per  cent.  This  latter  produces 
an  effervescence  in  acid  which  quickly  ends.  The  analysis 
of  a  loess  from  Kansas  City,  Missouri,  may  be  quoted  to 
show  the  general  chemical  composition. 

SiO,  A1,O3 Fe2O3 FeO  MgO  CaO  Na2O  K2O  H2O  CO2  XyO  Total 
74.5  12.3  3.3  0.1  1.1  1.7  1.4  1.8  2.7  0.5  0.4  =  99.$ 
XyO  -  minute  quantities  of  P  A.  TiO-j,  SO8  MnO  and  C. 


DESCRIPTION   OF   STRATIFIED  ROCKS  331 

Loess  occurs  in  widespread  areas  in  the  valley  of  the  Mississippi, 
In  the  states  of  Ohio,  Indiana,  Illinois,  Iowa,  Kansas,  Nebraska, 
Arkansas,  Missouri,  Tennessee,  Kentucky,  Alabama,  Louisiana, 
Mississippi,  and  Oklahoma.  It  is  found  also  in  Europe  in  various 
places,  especially  in  the  valleys  of  the  Rhine  and  its  tributaries, 
lying  in  isolated  patches  on  the  upper  hill  and  mountain  slopes  and 
in  the  same  way  in  the  Carpathians.  It  covers  an  enormous  area  in 
northern  central  China  with  thicknesses  attaining  1500-2000  feet, 
and  the  yellow  color  which  it  imparts  to  the  Hoang-ho  (Yellow  River), 
and  eventually  to  the  Yellow  Sea,  into  which  the  former  discharges, 
gives  to  these  their  names. 

It  is  now  generally  accepted  that  the  loess  is  an  seolian,  that  is, 
a  wind  blown  deposit  of  dust  which  has  accumulated  through  long 
periods  of  time.  This  is  shown  by  its  lack  of  stratification,  the 
spread  out  manner  in  which  it  lies  upon  the  surface,  filling  former 
inequalities,  the  remains  of  land  shells  which  are  found  in  it,  and 
by  the  small,  vertical  tubes  running  through  it  caused  by  the  roots 
and  stems  of  former  vegetation.  In  places,  however,  where  it  has 
been  washed  down  into  former  lakes,  ponds  and  streams,  it  becomes 
stratified.  In  America  and  Europe,  the  material  of  the  loess  is 
supposed  to  represent  the  finely  ground  rock  powder  of  the  glacial 
ice  sheet. 

A  characteristic  feature  is  the  common  occurrence  of  concretions 
of  carbonate  of  lime  and  of  oxide  of  iron.  They  often  assume  the 
odd  shapes  seen  in  the  flint  nodules  of  chalk.  The  perpendicular 
tubules  give  to  the  loess  a  vertical  cleavage,  which  produces  along 
river  banks  bold  bluffs. 

Adobe.  This  name  is  applied  to  a  very  fine-grained, 
coherent,  yet  friable  material  which  covers  wide  areas  in 
the  semi-arid  and  arid  regions  of  western  North  America, 
especially  in  the  southwestern  states  and  in  Mexico.  It 
resembles  loess  in  many  ways,  has  usually  the  harsh  feeling, 
when  rubbed  between  the  fingers,  and  is  of  a  yellowish, 
yellow-brown,  gray-brown  or  chocolate-brown  color.  Its 
use  in  the  form  of  sun-dried  brick  for  building  is  well- 
known.  It  is  the  result  of  the  finer  detritus  of  rock  decay 
on  the  higher  slopes  of  hills  and  mountains  accumulated 
on  the  lower  slopes,  plains,  valleys  and  basins,  in  part  by 
rain  wash,  and  in  part  by  the  action  of  the  wind  in  moving 
it  as  dust.  It  forms  a  valuable  soil  when  irrigated  and 
brought  under  cultivation. 


332  ROCKS  AND  ROCK  MINERALS 

Laterite.  This  is  a  red  soil  or  deposit  found  in  tropical 
regions  and  is  the  result  of  the  sub-serial  decay  of  many 
rocks,  especially  of  granite.  In  the  process  the  rocks  lose 
their  alkalies  and  alkali-earths  more  or  less  completely, 
and  there  remains  a  reddish,  cellular  mass,  consisting  of 
quartz  sand  mixed  with  clay-like  substance  (chiefly 
hydrargillite,  A1(OH)3)  with  iron  oxides  which  give  the 
color.  When  dried  it  may  become  very  hard  and  rock- 
like.  It  frequently  contains  concretions  of  the  iron  oxides. 

Loam.  The  common  arable  soils  of  the  greater  part  of 
the  world  are  comprised  under  this  heading.  Loam  con- 
sists of  a  mixture  of  sand  and  clay,  colored  yellow,  brown, 
or  reddish  by  iron  oxides,  or  dark  to  black  from  organic 
matter.  The  sandy  particles  are  chiefly  quartz,  often 
mingled  with  fragments  of  other  minerals.  On  rubbing 
between  the  fingers,  it  first  feels  harsh  from  the  gritty  sand 
particles;  if  the  rubbing  is  continued  and  these  are  allowed 
to  drop  out,  the  greasy  smooth  feeling  of  the  clay  is  finally 
perceived.  The  proportion  of  organic  matter  varies  very 
greatly;  the  black  soils  of  India  and  Russia  are  very  rich 
in  it. 


CHAPTER  X. 

THE  ORIGIN  AND  CLASSIFICATION  OF  META- 
MORPHIC  ROCKS. ' 

Introductory.  The  metamorphic  rocks  are  those  which, 
originally  sedimentary  or  igneous,  have  been  changed 
either  in  mineral  composition  or  in  texture,  or  in  both,  so 
that  their  primary  characters  have  been  altered,  or  even 
entirely  effaced.  Here  constantly,  as  elsewhere  in  geology, 
gradations  exist,  and  no  definite  line  can  be  drawn  on  the 
one  hand  between  the  sedimentary  rocks  and  their  meta- 
morpnio  products,  or  between  the  igneous  rocks  and  the 
metamorphic  ones  formed  from  them,  on  the  other.  Thus 
loose  chalks  pass  into  limestones,  and  these  into  crystalline 
marbles,  just  as  dolerites  merge  into  greenstones,  and  so 
on  into  hornblende  schists,  without  any  sharp  line  of 
domarkation.  But  there  comes  a  point  in  the  change  of 
each  original  rock,  either  of  composition  or  of  texture  and 
usually  of  both,  where  its  characters  and  relations  to  other 
rocks  have  become  so  individual  that,  for  practical  pur- 
poses, it  is  best  regarded  as  a  distinct  kind  of  rock.  Where 
this  line  shall  be  drawn  must  depend  upon  the  experience 
and  judgment  of  the  observer;  in  this  work  only  those 
cases  are  treated  where  the  change  has  been  so  defi- 
nite and  pronounced  as  to  produce  typical  metamorphic 
rocks. 

Rocks  for  the  most  part  are  composed  of  minerals,  and 
minerals  for  the  most  part  are  definite  chemical  combina- 
tions, which  are  only,  as  a  rule,  permanent  under  stable 
conditions.  If  the  minerals  are  submitted  to  new  condi- 
tions, quite  different  from  those  under  which  they  were 
formed,  with  new  chemical  and  physical  factors  operating 
upon  them,  they  will  tend  to  change  into  other  minerals, 

833 


334  ROCKS  AND  ROCK  MINERALS 

that  is,  to  turn  into  new  chemical  combinations,  which  will 
be  the  most  stable  under  the  new  conditions.  A  familiar 
example  is  the  decay  of  the  feldspar  of  igneous  rocks,  and 
its  change  into  clay  and  other  substances  through  the 
action  of  water  and  carbon  dioxide,  as  treated  under 
granite.  The  change  in  conditions  may  be  so  slight  that 
some  rock  minerals  may  be  able  to  resist  them  indefinitely, 
while  others  less  stable  may  succumb.  Thus  igneous 
rocks,  formed  by  the  cooling  and  crystallization  of  molten 
magmas,  may  remain  in  the  depths  for  millions  of  years, 
and  on  coming  to  the  surface  through  erosion  and  denuda- 
tion, may  be  found  entirely  unchanged,  or  with  only  one 
or  two  of  the  constituent  minerals  altered.  At  the  surface 
they  are  at  once  subjected  to  new  conditions,  to  the  com- 
bined effects  of  changes  of  temperature,  to  moisture,  the 
various  gases  of  the  atmosphere,  the  products  of  organic 
life,  etc.,  and  they  commence  to  break  up  and  to  form  into 
new  compounds.  Then  their  ultimate  conversion  is  only 
a  question  of  time.  The  same  is  true  of  the  sedimentary 
rocks,  only  in  lesser  degree.  They  are  formed  of  mineral 
particles,  deposited  in  water  and,  usually,  cemented  by 
pressure  and  deposits  from  solution.  While  they  remain 
deeply  buried  and  under  fairly  stable  conditions,  they  are 
unchanged;  when  they  are  exposed  to  the  atmosphere 
they  also  tend  to  change  and  decay,  especially  in  those 
minerals  that  are  susceptible. 

All  these  changes  which  occur  upon  the  surface  are 
strictly  to  be  classed  as  metamorphic  ones,  and  the  prod- 
ucts, in  a  geologic  sense,  are  metamorphic  rocks.  But 
for  practical  purposes  all  these  materials  formed  by  the 
action  of  weathering  and  by  the  decay  of  rocks  on  or  near 
the  surface,  such  as  the  soils,  are  not  here  included.  They 
have  been  previously  mentioned  under  the  foregoing  rock 
types,  so  far  as  seems  desirable  for  the  object  of  this  work, 
and  only  those  rocks  are  treated  as  metamorphic  which, 
while  buried  at  depth  below  the  surface,  have  suffered, 
through  the  action  of  certain  agencies  to  be  presently 


ORIGIN  OF  THE  METAMORPHIC  ROCKS          335 

described,  changes,  which  have  practically  converted 
them  into  new  kinds  of  rocks. 

Metamorphic  Agencies.  The  chief  metamorphic  agen- 
cies are  mechanical  movements  of  the  earth's  crust  and 
pressure,  the  chemical  action  of  liquids  and  gases,  and  the 
effect  of  heat.  We  may  simplify  these  into  the  effects  of 
movement,  water  solutions,  and  heat,  and  all  three  of 
these  are  required  to  produce  complete  metamorphism 
in  rocks,  though  not  necessarily  all  to  the  same  extent, 
since  sometimes  one  factor  is  more  predominant,  and 
sometimes  another.  Thus  in  the  metamorphism  which 
has  been  already  described  as  contact  metamorphism, 
induced  by  the  intrusion  of  a  body  of  magma,  the  effect 
of  heat  is  the  most  important,  that  of  gases  and  liquids 
less  so,  while  the  effect  of  movements  of  the  crust,  or 
pressure,  is  negligible.  The  rocks  produced,  however,  are 
actually  metamorphic,  but  for  practical  reasons  they  have 
been  given  separate  consideration,  and  are  not  included 
among  these  under  treatment.  We  will  consider  the 
different  agencies  separately. 

Movement  and  Pressure.  Pure  simple  downward  pres- 
sure, to  the  amount  exerted  in  the  upper  part  of  the  earth's 
outer  crust,  appears  to  have  little  metamorphic  effect.  It 
tends  without  doubt  to  consolidate  the  material  of  sedi- 
ments by  bringing  the  grains  closer  together,  but  many 
instances  may  be  cited  of  sediments,  buried  under  great 
thicknesses  of  deposits  for  geologic  ages,  which  on  being 
raised  and  exposed  by  erosion  without  disturbance,  such 
as  folding,  are  found  to  be  practically  in  unchanged 
condition. 

On  the  other  hand,  as  commonly  supposed,  through  the 
gradual  contraction  of  the  earth,  the  outer  crust  is  under 
compression,  and  this  finds  relief  from  time  to  time  by 
buckling  or  wrinkling  up  of  the  outer  shell  into  mountain 
ranges.  This  compressive  force,  thus  acting  with  lateral 
thrust,  is  therefore  spoken  of  as  orogenic,  i.e.,  mountain 
forming.  By  it  whole  masses  of  strata  with  possibly 


336  ROCKS  AND  ROCK  MINERALS 

included  igneous  rocks  —  intrusive,  extrusive  and  frag- 
mental  volcanic  —  are  folded,  crushed,  and  mashed 
together  in  the  most  involved  and  intricate  manner.  Not 
only  are  the  rocks  then  subjected  to  vast  pressure,  but 
they  are  also  subjected  to  enormous  shearing  stresses, 
which  tend  to  produce  forced  differential  movements 
among  the  rock  particles.  It  is  particularly  this  latter 
effect  which  is  of  great  potency  in  producing  meta- 
morphism  Its  effects  may  often  be  seen  megascopic  ally 
by  the  manner  in  which  large  cryrjtak,  included  pebbles, 
or  fossils  are  flattened  and  elongated,  or  broken  into  frag- 
ments which  are  drawn  out  into  thin,  lenticular  masses 
in  the  direction  of  shear.  The  microscope  shows  that 
even  minute  crystals  are  broken,  and  their  optical  proper- 
ties affected,  as  the  result  of  the  strain.  It  is  possible 
indeed,  for  this  agency  working  alone  to  produce  rocks 
having  the  characteristic  outward  metamorphic  texture, 
without  any  change  in  their  ^riginal  mineral  composition, 
but  in  combination  with  heat  and  waier,  it  is  of  the  highest 
importance  in  inducing  chemica*  changes,  and  the  produc- 
tion of  new  minerals.  It  is  indeed  a  noticeable  fact  that 
so  long  as  the  rocks  retain  their  original  position,  "hey  are 
unaltered,  but  as  we  commence  to  find  them  disturbed 
by  erogenic  forces,  they  begin  to  show  signs  of  meta- 
morphism,  and  in  proportion  to  the  degree  to  which  they 
have  been  folded  up,  mashed,  and  sheared,  they  become 
more  and  more  metamorphosed. 

Heat.  The  effect  of  heat  as  a  metamorphic  agent  is  very 
powerful,  as  is  so  well  shown  in  local  or  contact  meta- 
morphism.  It  increases  very  greatly  the  solvent  action 
of  solutions;  it  tends  in  many  cases  to  break  up  existing 
chemical  compounds  which  form  minerals,  and  to  promote 
new  chemical  arrangements.  The  heat  needed  for  meta- 
morphism  may  come  from  the  interior  of  the  earth,  which 
increases  greatly  with  the  depth;  it  may  be  supplied  in 
part  by  the  transformation  of  energy  resulting  from  the 
movements,  the  folding  and  crushing  of  the  rock  masses. 


ORIGIN  OF  THE  METAMORPHIC  ROCKS          337 

and  in  part  it  may  result  from  intrusions  of  molten  magma, 
which  are  very  liable  to  rise  and  invade  the  rock  masses 
as  they  are  uplifted  and  folded. 

Liquids  and  Gases.  The  chief  of  these  is  of  course 
water,  which  under  heat  and  pressure  becomes  a  powerful 
chemical  agency.  It  acts  as  a  solvent,  and  promotes 
recrystallization,  and  taking  part  in  the  chemical  compo- 
sition of  some  of  the  minerals,  such  for  example  as  micas 
and  epidote,  it  is  a  substance  necessary  to  their  formation. 
It  is,  without  doubt,  aided  also  in  its  action  by  substances 
it  may  carry  in  solution,  such  as  alkalies,  and  by  volatile 
emanations  coming  from  magmatic  intrusions,  like  boric 
acid,  fluorine,  etc.,  as  already  explained  under  contact 
metamorphism.  It  is  this  which  explains  the  presence 
in  metamorphic  rocks  of  such  minerals  as  tourmaline, 
chondrodite,  and  vesuvianite,  which  are  characteristic  of 
pneumatolytic  contacts,  and  of  micas,  hornblendes  and 
other  minerals  which  contain  fluorine. 

Effect  of  Depth.  The  outer  crust  of  the  earth  has  been 
divided  by  geologists  into  different  zones,  according  to 
the  various  geological  processes  at  work.  In  the  outer- 
most one,  down  to  the  level  at  which  ground  water  stands, 
the  rocks  are  full  of  fractures,  and  are  exposed  to  atmos- 
pheric agencies  —  moisture,  carbon  dioxide,  oxygen,  etc. 
In  this  the  rocks  tend  to  decay,  to  be  converted  into  car- 
bonates and  hydroxides,  and  to  form  soils.  It  is  called 
the  belt  of  weathering,  and  is  the  one  of  rock  destruction. 
Below  this  lies  another,  in  which  the  rocks  are  also  full  of 
fractures  and  cavities  filled  with  water.  Its  upper  level 
is  that  of  ground  water;  below,  it  reaches  to  the  point 
where  the  pressure  of  the  superincumbent  masses  and  the 
contraction  of  the  crust  becomes  so  great  that  all  fractures 
and  openings  are  closed  up,  since  the  stress  is  so  much 
greater  than  the  strength  of  the  rocks,  that  they  crush 
under  it,  and  are  to  be  regarded  as  being  in  a  relatively 
plastic  state.  In  this  zone  the  chemical  action  of  water 
is  most  important,  aided  by  the  substances  it  may  carry 


338  ROCKS  AND  ROCK  MINERALS 

in  solution.  The  tendency  is  to  change  the  minerals  to 
hydrates,  and  to  a  lesser  amount  to  carbonates;  thus 
oUvine,  an  anhydrous  silicate,  becomes  converted  into 
the  hydrous  silicate,  serpentine.  Substances  are  taken 
into  solution  and,  reinforced  by  those  leached  out  from 
the  belt  above  and  carried  down,  are  deposited  in  the 
pores  and  fissures  of  the  rocks;  hence  it  is  called  by  Pro- 
fessor Van  Hise  the  belt  of  cementation,  because  the  rock- 
grains  are  thus  cemented  together. 

Below  this  lies  the  zone  where,  as  stated  above,  the 
pressure  becomes  so  great  that  all  openings  are  closed  up, 
and  the  rocks  may  be  regarded  as  in  a  plastic  condition. 
Its  upper  level  is  variable  and  depends  on  geological  con- 
ditions; in  times  of  quiet  it  may  be  as  deep  as  six  miles 
below  the  surface;  in  times  of  mountain  making,  it  may 
rise  much  higher  than  this.  Of  what  may  be  its  lower 
level,  we  know  nothing.  In  this,  the  chief  agencies  are 
the  enormous  pressure  and  the  increasing  heat  of  the 
earth;  the  r61e  played  by  liquids  and  volatile  substances 
is  of  less  importance;  the  tendency  is  for  them  to  be  gotten 
rid  of,  to  be  squeezed  out.  The  chief  work  done  in  this 
zone  is  molecular  rearrangement,  in  which  less  stable 
mineral  compounds  are  broken  up,  and  new  ones  of  higher 
specific  gravity  and  smaller  volume,  through  condensation, 
are  formed.  Carbonates  are  converted  'nto  silicates  and 
the  carbon  dioxide  expelled;  hydrated  minerals  have 
their  water  driven  out  and  new  minerals,  with  less  or  no 
water,  are  formed.  This  zone  of  rock  flowage,  in  contrast 
to  the  zone  of  fracture  above  it,  has  been  called  the  zone  of 
anamorphism  by  Professor  Van  Hise.  We  may  term  it 
the  zone  of  constructive  metamorphism. 

It  is  chiefly  in  the  lower  part  of  the  belt  of  cementation 
(zone  of  fracture),  and  the  upper  part  of  the  zone  of  rock 
flowage, that  the  greater  part  of  the  work  of  metamorphism, 
in  the  production  of  the  metamorphic  rocks  as  we  see 
them,  is  done.  In  the  upper  zone,  the  results  are  chiefly 
those  produced  by  dynamic  shearing,  and  the  imposing 


ORIGIN  OF  THE  METAMORPHIC  ROCKS          339 

upon  the  rocks  of  characteristic  textures.  Chemical 
work  may  be  done  and  new  minerals  produced,  but  it  is 
possible  for  new  textures  to  be  formed  without  change 
in  mineral  composition.  In  the  lower  zone,  the  work  done 
is  largely  chemical,  new  and  more  stable  mineral  combi- 
nations being  formed;  and  here  also  characteristic  tex- 
tures are  produced. 

Minerals  of  Metamorphic  Bocks.  Just  as  certain  min- 
erals, of  which  nephelite  and  sodalite  might  be  mentioned 
as  examples,  are  characteristic  of  igneous  rocks,  so  other 
minerals  are  peculiar  to  the  metamorphic  ones,  such  as 
cyanite,  zoisite,  staurolite  and  talc.  Other  minerals  are 
found  in  both  groups  alike,  such  as  quartz,  feldspar,  horn- 
blende, pyroxene,  garnet  and  mica.  It  should  be  remem- 
bered, however,  that  the  names  just  mentioned  are  really 
names  of  families,  under  which  quite  a  variety  of  individual 
mineral  species  may  be  grouped,  on  account  of  certain 
common  properties,  such  as  crystal  form.  Thus  in  the 
hornblende  group,  arfvedsonite  is  found  only  in  igneous 
rocks;  tremolite  and  uralite  occur  practically  only  in  meta- 
morphic ones;  common  hornblende  occurs  in  both.  Of  the 
pyroxenes,  the  normal  home  of  augite  is  in  igneous  rocks, 
of  wollastonite,  a  pyroxene-like  mineral  of  the  composition 
CaSi03,  in  the  metamorphic  ones;  common  pyroxene  in 
both.  Of  the  micas,  paragonite  has  been  found  only  in 
metamorphic  schists,  biotite  and  muscovite  are  present 
in  both  groups  of  rocks,  but  muscovite  is  relatively  rare  in 
fresh,  normal,  igneous  ones.  In  the  garnet  group,  pyrope, 
the  magnesia-alumina  garnet,  is  formed  only  in  igneous 
rocks  very  rich  in  magnesia  and  low  in  silica,  such  as  the 
peridotites;  H  occurs  m  them,  or  in  the  serpentines  formed 
from  them,  while  grossularite,  the  lime-alumina  garnet, 
has  its  characteristic  home  in  metamorphic  limestones; 
almandite  and  common  garnet  are  found  both  in  igneous 
and  metamorphic  rocks.  In  the  following  list  are  given 
the  minerals  which  may  occur  in  metamorphic  rocks;  the 
first  column  contains  those  of  wide  distribution,  and  of 


340  ROCKS  AND  ROCK  MINERALS 

prime  importance,  as  chief  components;  the  second  column, 
those  of  lesser  importance,  which  occur  either  as  prominent 
accessory  minerals,  or  locally  developed  as  chief  compo- 
nents; the  third,  occasional  minerals,  which  may  be  at 
times  megascopically  developed.  But  this  is  true  only 
in  a  general  way,  and  over  emphasis  must  not  be  laid  on 
these  divisions. 

I  II  III 

Quartz  Garnets  Graphite 

Feldspars  Staurolite  Tourmaline 

Biotite  Epidote  Chrondrodite 

Muscovite  Zoisite  Vesuvianite 

Hornblendes  Cyanite  Hematite 

Calcite  Pyroxenes 

Dolomite  Magnetite 

Chlorite  Talc 
Serpentine 

Of  these  minerals,  chlorite,  serpentine,  and  talc  are 
specially  characteristic  of  the  upper  zone,  while  cyanite, 
staurolite,  and  some  of  the  others  are  formed  in  the  lower 
zone.  Some  minerals,  like  quart:',  and  some  members  of 
the  groups  may  be  formed  in  either  zone,  or  be  persistent 
components  of  the  original  rocks. 

Texture  of  Metamorphic  Rocks.  The  metamorphic 
rocks  resemble  the  greater  part  of  the  igneous  ones,  in 
that  they  possess  a  highly  crystalline  character,  so  much 
so  that  they  are  frequently  referred  to  as  the  crystalline 
schists.  On  the  other  hand,  they  resemble  the  stratified 
ones  in  possessing  a  parallel  structure  which  may  closely 
resemble  stratification.  Thus  they  show  analogies  to 
both  of  the  other  great  rock  groups.  This  parallel  structure 
expresses  itself  to  a  greater  or  lesser  degree  by  a  foliated, 
laminated,  or,  as  it  is  frequently  termed,  a  schistose  texture, 
one  in  virtue  of  which  the  rock  tends  to  split  or  cleave 
more  or  less  perfectly  in  the  direction  of  a  certain  plane 
passing  through  it.  This  direction  of  cleavage  is  called 
the  chief  fracture,  and  the  break  of  the  rock  at  right  angles 


ORIGIN  OF  THE  METAMORPHIC  ROCKS         341 

to  it  is  termed  the  cross  fracture.  Highly  crystalline  rocks 
exhibiting  this  texture  are  called  gneisses  or  schists,  ac- 
cording to  their  mineral  composition,  as  described  later. 
While  it  is  the  characteristic  texture  for  the  metamorphic 
rocks,  there  are  a  few,  such  as  serpentine,  marble 
and  quartzite,  that  for  certain  reasons  to  be  explained/ 
may  not  show  any  trace  of  it,  and  yet  are  true  meta- 
morphic rocks. 

Observation  of  the  gneisses  and  schists  shows  that  this  texture 
is  due  to  arrangements  of  unlike  mineral  grains  in  layers,  or  very 
flat  lenses,  or  to  a  parallel  arrangement  of  minerals  having  prismatic 
or  tabular  forms,  such  as  hornblende  or  mica,  or  to  a  mixture  of 
both.  It  is  a  result  of  the  orogenic  forces,  the  shearing  and  pressures 
to  which  the  original  rocks  have  been  subjected,  and  it  makes  no 
difference  whether  these  were  igneous  or  sedimentary,  this  texture 
may  be  imposed  upon  both  alike  under  proper  conditions.  The 
superficial  resemblance,  which  the  gneisses  and  schists  bear  to  strati- 
fied rocks  in  their  parallel  laminated  character,  for  a  long  time  led 
geologists  to  think  that  the  former  must  have  been  derived  wholly 
from  the  latter,  and  the  general  recognition  that  they  contain  former 
igneous  ones  as  well  has  come  only  in  the  last  twenty-five  years 
through  petrographic  and  chemical  studies.  From  the  fact  that  in 
places  stratified  rocks  could  be  traced  into  metamorphic  ones  and 
the  latter  into  igneous  ones,  it  was  even  assumed  that  the  igneous 
rocks  were  in  part  derived  from  sediments  by  extreme  metamorphism. 
Such  cases  merely  represent  instances  where  both  have  been  meta- 
morphosed in  common,  with  a  remnant  at  either  end  which  is  not 
metamorphosed,  and  whose  original  characters  may  therefore  be 
recognized.  In  the  light  of  our  present  knowledge  we  should  be 
no  more  justified  in  tracing  out  such  a  deduction,  than  we  would  in 
reversing  it,  and  deriving  the  stratified  rocks  from  the  igneous  ones 
by  metamorphic  processes! 

Varieties  of  Texture,  Three  chief  varieties  of  the  schis- 
tose texture  may  be  recognized,  (1)  the  banded,  in  which 
unlike  mineral  layers  are  in  parallel  bands,  as  shown  in 
Fig.  A,  Plate  34.  This  resembles  stratification,  but  may 
be  induced  in  igneous  masses  as  the  result  of  shear.  (2) 
the  lenticular,  or  foliated,  in  which  some  of  the  components 
are  collected  into  thinner  or  thicker  lenses,  around  which 
the  other  minerals  tend  to  be  wrapped  or  wound,  as.  shown 


342  ROCKS  AND  ROCK  MINERALS 

in  Fig.  B,  Plate  34,  which  shows  a  view  of  the  cross  frac- 
ture. The  surface  of  chief  fracture  in  this  case  is  apt  to 
be  more  or  less  lumpy,  and  not  to  show  well  the  minerals 
of  the  lenses  Both  this  and  the  foregoing  variety  vary 
greatly  from  coarse  to  fine.  (3)  The  slaty  texture  is  one 
in  which  the  mineral  grains  are  extremely  small,  usually 
too  small  to  be  seen  with  the  eye,  and  often  even  with  the 
lens;  the  rocks  appear  dense,  but  they  have  the  capacity 
of  splitting  into  thin  slabs,  as  seen  in  roofing  slates. 
The  cause  of  this  is  discussed  under  the  description  of 
slates. 

Metamorphic  rocks  frequently  contain  large  and  well- 
developed  crystals  of  minerals,  which  have  formed  as  a  re- 
sult of  the  processes  to  whichthe  rocks  have  been  subjected. 
These  may  be  very  much  greater  in  size  than  the  average 
grain  of  the  rock,  and  this  contrast,  together  with  the  per- 
fection of  their  crystal  form,  produces  a  strong  analogy 
to  the  porphyritic  texture  of  igneous  rocks.  They  are  not 
true  porphyries,  however,  not  only  because  the  texture 
is  not  of  igneous  origin,  but  also  because  these  large  crystals 
are  not  of  an  older  generation,  but  are  actually  of  later 
formation  than  the  minerals  of  the  apparent  groundmass 
in  which  they  lie.  It  is  therefore  termed  the  pseudo- 
porphyritic  texture.  That  these  minerals  or  pseudo-pheno- 
crysts  are  of  later  formation  is  shown  by  the  fact  that  they 
frequently  contain  as  inclusions  the  other  rock  minerals, 
and  sometimes  the  inclusions,  such  as  bits  of  quartz,  graph- 
ite, etc.,  pass  through  the  large  crystal  in  the  lines  of 
original  stratification,  and  out  beyond  it.  Moreover,  it 
may  be  frequently  noticed  that  where  these  pseudo-pheno- 
crysts  are  not  equi dimensional,  but  elongated,  they  may  lie 
in  the  rock  pointing  in  all  directions;  their  longer  axes  do 
not  necessarily  lie  in  the  direction  of  schistosity,  like  those 
older  minerals,  which  have  been  arranged  by  the  pressure 
and  shearing.  Having  grown  in  the  zone  of  pressure, 
they  are  not  oriented  by  it,  unless  subsequent  and  later 
movement  and  shearing  should  take  place  after  their  for- 


PLATE   34. 


A.    BANDED    GNEISS. 


B.    LENTICULAR   OR   FOLIATED   GNEISS. 
(Maryland  Geological  Survey.) 


ORIGIN  OF  THE  METAMORPHIC  ROCKS          343 

mation.  The  space  in  the  rock  in  which  movement  of 
material  goes  on  to  produce  these  larger  crystals  is  clearly 
shown  in  Plate  35,  which  is  a  photograph  of  a  garnet 
in  gneiss.  Around  the  garnet  is  a  zone  of  feldspar, 
from  which  all  the  ferromagnesian  minerals,  visible  be- 
yond it,  have  disappeared,  having  been  used  up  in  its 
formation. 

The  crystals  described  above  should  not  be  confused 
with  larger  crystals  or  crystal  masses  in  the  rock,  which 
may  also  give  it  a  porphyritic,  appearance,  but  which  are 
really  remains  of  former  structures.  Such  may  be  former 
phenocrysts  of  some  porphyritic,  igneous  rock,  or  large 
grains  from  some  former  coarse-granular  igneous  rock,  or  a 
pebble  from  a  conglomerate.  They  are  apt  to  form  ovoid 
masses,  and  they  are  then  really  a  pronounced  case  of  the 
lenticular  texture,  which  is  sometimes  termed,  following 
the  German  name,  augen  (eye)  texture. 

Relation  to  Previous  Textures,  etc.  In  proportion  to  the 
degree  of  metamorphism  which  rocks  have  suffered  do  we 
find  that  the  characteristic  textures  described  above  have 
been  imposed  upon  them.  But  not  infrequently,  as 
though  looking  through  the  veil  which  metamorphism 
has  cast  over  them,  we  can  see  back  of  these  features 
remains  of  original  textures  and  structures  which  are 
characteristic  of  igneous  and  sedimentary  rocks.  Thus, 
as  indicated  above,  we  may  see  that  the  original  texture 
was  that  of  a  porphyry,  or  we  may  find  remnants  of  the 
spherulites,  lithophysae,  and  flow  lines  of  some  felsite 
lava,or  of  the  amygdules  of  some  basaltic  one;  on  the  other 
hand,  ovoid  masses  of  different  mineral  composition  may 
indicate  a  former  conglomerate,  or  parallel  layers,  differing 
in  general  mineral  and  chemical  composition,  may  show 
former  stratified  material.  Such  indications  may  be  very 
useful  in  ascertaining  the  former  origin  of  a  metamorphic 
rock,  and  in  some  cases  may  positively  identify  it,  but 
deductions  from  this  source  should  always  be  made  tenta- 
tively, and  used  with  great  caution,  for  there  are  many  con- 


344 


ROCKS  AND  ROCK  MINERALS 


fusing  appearances  of  this  kind  which  may  lead  to  serious 
error,  unless  they  are  checked  by  microscopic  examination 
and  chemical  analyses. 

Chemical  Composition.  The  chemical  composition  of 
the  metamorphic  rocks  is  extremely  variable,  and  it  is 
evident  that  this  must  be  the  case,  when  one  considers  the 
heterogeneous  materials  from  which  they  may  be  derived. 
If  we  take  them  together,  as  a  class  of  rocks,  the  com- 
position, therefore,  cannot  have  the  significance  which  it 
plays  in  the  igneous  ones,  in  showing  their  mutual  rela- 
tions. It  may,  however,  be  of  great  importance  as  an  aid 
in  helping  to  determine  their  origin.  Thus,  in  examining 
the  chemical  analysis  of  a  metamorphic  rock,  we  may  be 
able  to  say  that  it  is  similar  to  those  of  known  igneous 
rocks  and  it  may  therefore  have  been  originally  of 
igneous  nature,  and  on  the  other  hand  the  analysis  may 
show  definitely  that  it  could  not  have  been  any  igneous 
rock,  and  consequently  it  must  have  been  of  sedimentary 
origin. 


SiO2 

A12O3 

Fe2O3 

FeO 

MgO 

CaO 

Na2O 

K2O 

H2O 

XyO 

Total 

I... 

75.7 

13.2 

1.8 

0.4 

0.6 

2.1 

4.7 

1.0 

99.5 

II.. 

50.3 

14.1 

7.0 

5.3 

7.2 

8.1 

4.0 

2.3 

1.6 

_ 

99.9 

III. 

74.7 

8.9 

9.6 

— 

1.9 

1.1 

0.4 

1.0 

1.1 

0.5 

99.2 

I,  Gneiss,  near  Freiberg,  Saxony;  IT,  Hornblende  Schist  (amphi- 
bolite)  Vestana,  Sweden;  III,  Gneiss,  near  Rawdon,  Quebec. 

Thus,  in  the  analyses  given  above,  that  of  No.  1  might  well  be  of  an 
ordinary  granite,  as  may  be  seen  by  reference  to  those  given  under 
granite;  it  might  also,  however,  be  that  of  an  arkose  derived  from  such 
a  granite.  No.  II  has  the  composition  of  a  gabbro;  it  might  have 
been  such  a  rock  originally,  or  a  dolerite,  or  basalt ;  it  does  not  suggest 
any  ordinary  sedimentary  rock.  No.  Ill  on  the  other  hand  has  no 
analogy  among  igneous  rocks;  the  alkalies  and  alumina  are  too  low 
for  the  silica  and  the  ferric  oxide  too  high;  it  must  be  of  sedimentary 
origin  and  suggests  an  impure,  ferruginous  sandstone. 


ORIGIN  OF  THE  METAMORPHIC  ROCKS         345 

It  is  inferred,  of  course,  that  while  movement  among  the 
molecules  within  limited  distances  has  occurred,  whereby 
exchanges  among  the  oxides  are  produced,  involving 
recrystallization  and  the  formation  of  new  mineral  com- 
pounds, the  chemical  composition  of  a  rock  mass  as  a 
whole  has  remained  unaltered.  That  this  is  so,  is  shown 
by  the  fact,  that  in  innumerable  occurrences  stratified 
rocks,  although  utterly  changed  in  mineral  composition 
from  their  former  state,  still  retain  the  spacing  and  relative 
volume  relations  of  the  strata  which  they  originally  had. 
Thus  one  band  of  strata,  perhaps  only  a  fraction  of  an 
inch  in  thickness,  is  sharply  marked  off  by  its  grain, 
minerals,  and  texture  from  those  above  and  below  it. 
There  has  been  no  melting  and  no  formal  transfusion  of 
substance,  consequently  the  changes  which  have  occurred 
are,  so  to  speak,  inward,  those  which  lie  within  the  range 
of  molecular  attraction.  To  this  general  statement  that 
there  is  no  change  in  mass  composition  in  metamorphism, 
there  is  one  exception,  and  that  is,  that  volatile  substances, 
liquids  and  gases,  may  be  driven  out  and,  conversely,  new 
ones  may  enter  and  pass  into  mineral  combinations,  as 
previously  explained  under  the  action  of  liquids  and  gases 
as  agents.  This  is  most  strikingly  seen,  perhaps,  in  the 
metamorphism  of  impure  limestones,  as  described  in  the 
section  dealing  with  marble,  and  is  thoroughly  analogous 
to  what  has  already  been  stated  under  contact  metamor- 
phism. 

Injection  of  Gneisses  and  Schists.  It  has  been  pre- 
viously mentioned  that  part  of  the  heat  of  metamorphism, 
and  of  the  liquids  and  gases  involved  in  the  production 
of  minerals,  is  supplied  by  intrusions  of  igneous  magma, 
which  are  particularly  liable  to  rise  and  invade  those 
areas  where  crustal  movements  are  starting  metamorphic 
agencies  at  work.  In  such  areas,  of  course,  the  effect  of 
contact  merges  into  that  of  general  metamorphism  and  no 
definite  line  can  be  drawn  between  them.  Indeed  the 
earlier  formed  intrusions  may  themselves  become  more 


346  ROCKS  AND  ROCK  MINERALS 

or  less  metamorphosed,  or  have  metamorphic  textures 
imposed  upon  them  by  repetitions  of  the  processes, 
and  this  may  happen  while  they  are  in  a  solid,  or 
yet  partly  plastic,  condition.  There  is,  however,  another 
function  which  t'hese  magmas,  rising  under  great  pressure 
into  rocks  already  schistose  and  foliated,  may  perform; 
they  may  squeeze  themselves  in  thin  veins,  sheets,  and 
lenticles  into  the  schists  surrounding  them,  so  that  these 
rocks  may  become  partly  igneous,  partly  metamorphic,  in 
composition.  And,  as  previously  explained  under  con- 
tact metamorphism  and  pegmatite  dikes,  these  effects 
may  be  greatly  aided  by  liquid  and  gaseous  emanations 
from  the  magma  masses.  This  process  has  been  termed 
the  injection  of  schists  by  magmatic  material  and,  although 
as  it  has  been  doubted  by  some  geologists,  just  as  it  has 
been  given  entirely  too  general  an  application  by  others,  it 
has  undoubtedly  occurred  in  many  places.  By  it  we  are 
enabled  to  understand  the  veins,  stringers,  and  lenses  of 
granite  penetrating  the  schists  in  the  neighborhood  of 
larger  granite  intrusions  in  many  places,  which  would  be 
otherwise  incomprehensible.  In  this  connection  also,  it 
should  be  remembered  that  the  intrusive  effects  in  the 
lower  zone  of  rock  flowage  may  be  expected  to  be  quite 
different  from  those  in  the  upper  zone  of  rock  fracture. 

Occurrence  and  Age.  The  metamorphic  rocks  have  a 
wide  distribution  over  the  earth's  surface,  and  in  many 
places  they  occupy  great  areas,  over  which  they  are  the 
*>nly  ones  exposed.  There  is  good  reason  also  for  be- 
lieving that  they  form  the  basement  upon  which  all  the 
later  unmetamorphosed,  sedimentary  rocks  rest.  The 
reason  for  this  is,  that  wherever  these  later  strata  are 
sufficiently  eroded  away,  this  metamorphic  basement  has 
come  to  light.  The  only  exception  to  this  general  dis- 
tribution over  the  continental  areas  is  in  those  places 
where  later  intrusions  of  igneous  magmas  have  come  up 
through  them  and  are  now  exposed  as  bathyliths,  stocks, 
dikes,  etc.  But  these  constitute  but  a  subordinate  part 


ORIGIN  OF  THE   METAMORPHIC  ROCKS         347 

of  the  total  area.  It  is  their  extension  over  such  wide 
areas  which  has  led  to  the  processes,  that  have  produced 
them,  being  called  regional  metamorphism,  in  contrast  to 
the  forming  of  the  small  zones  around  intrusive  igneous 
masses,  which  is  therefore  termed  local  metamorphism. 
There  is  no  difference  in  principle,  however,  between  these 
two,  only  in  the  relative  intensity  with  which  the  varied 
agents  have  operated.  The  metamorphic  rocks  are  found 
also  in  folded  mountain  ranges,  of  which  they  form  the 
interior  core,  and  which  subsequent  erosion  brings  to  light. 
In  proportion  to  the  intricacy  of  the  folding  and  mashing 
of  the  strata,  so  is  the  degree  of  metamorphism  increased. 
This  is  so  well  established,  that  when  we  find  areas  where 
the  rocks  are  intricately  folded  and  completely  meta- 
morphic, but  not  of  any  great  elevation,  we  assume  that 
such  an  elevation  formerly  existed,  but  has  been  eroded 
away,  or  in  general  that  metamorphic  rocks  can  only 
become  exposed  at  the  surface  through  erosive  processes. 
It  is  these  facts  that  led  to  the  view,  formerly  held,  that 
metamorphic  rocks  must,  geologically  speaking,  be  of 
very  great  age.  This  is,  however,  by  no  means  necessarily 
the  case.  For,  on  the  one  hand,  we  find  unmodified 
sands  of  Cambrian  age  in  eastern  Russia,  and  unaltered 
beds  of  Ordovician  age  in  the  upper  Mississippi  valley, 
which  have  not  been  changed  from  their  original  position, 
while  on  the  other,  strongly  folded  strata  of  Tertiary  age 
in  the  Coast  Range,  in  the  Alps,  and  in  other  mountains, 
are  in  places,  profoundly  metamorphosed.  Rocks  that  are 
metamorphic  are  likely  to  be  old,  but  not  necessarily  so, 
just  as  a  battle-scarred  soldier  is  likely  to  be  a  veteran, 
rather  than  a  recent  recruit.  It  merely  depends  on 
whether  they  have  been  subjected  to  metamorphic 
processes  or  not,  and  the  older  they  are,  the  more  likely 
they  are  to  have  suffered  from  them.  Time,  however, 
is  one  of  the  great  factors  in  metamorphism,  and  even  in 
the  recent  strata  which  have  been  changed,  the  time  in- 
volved, from  our  standpoint,  is  very  long. 


348 


ROCKS  AND   ROCK  MINERALS 


Classification  of  Metamorphic  Rocks.  It  would  be 
natural  to  classify  the  metamorphic  rocks  according  to 
the  origin  of  their  material,  and  to  separate  those  of  igneous 
from  those  of  sedimentary  formation.  In  some  cases  this 
may  be  done.  Thus  it  is  clear  that  marble  is  not  of  igneous 
origin,  but  when  we  attempt  to  carry  this  principle 
through,  it  quickly  becomes  impracticable,  especially 
if  we  can  use  only  megascopic  means  of  determination. 


SiOz 

A12O3 

FezOa 

FeO 

MgO 

CaO 

Na^O 

K2O 

H20, 
etc. 

Total 

69.9 

13.1 

2.5 

0.7 

trace 

3.1 

5.4 

3.3 

1.0 

99.0 

69.9 

14.9 

1.8 

0.6 

0.6 

1.5 

5.3 

3.9 

1.3 

99.8 

Thus  in  the  two  analyses  given  above,  the  upper  one  is 
that  of  the  Portland  sandstone  of  Connecticut,  a  fine- 
grained arkose  full  of  feldspar,  the  lower  one  that  of  an 
intrusive  granite  porphyry  from  the  Crazy  Mountains, 
Montana.  It  is  evident  that  if  these  two  rocks,  one  sedi- 
mentary, the  other  igneous,  should  be  so  thoroughly 
metamorphosed  as  to  lose  all  traces  of  their  original 
textures,  it  would  be  impossible  to  discriminate  them  from 
one  another,  or  to  say  what  their  original  status  was. 

Remembering  the  simple  primary  classification  of  the 
sedimentary  rocks  previously  given,  it  is  possible,  in  a  very 
general  way,  to  show  the  relation  between  the  most  com- 
mon ones  and  their  metamorphic  derivatives  in  the  fol- 
lowing table: 


Sediments. 

Compacted  Strata. 

Metamorphic  Rocks. 

Gravel  

Conglomerate 

Gneiss  and  various  schists 

Sand  

Sandstone       .    . 

Quartzite        "               " 

Silt  and  Clay  

Shale     

Slate                "               " 

Lime  deposits  

Limestone  

Marble            "              " 

ORIGIN  OF  THE  METAMORPHIC  ROCKS          349 

In  the  case  of  the  igneous  rocks,  recalling  that  they  may 
be  roughly  divided  into  two  main  groups,  the  one  chiefly 
feldspathic,  and  the  other  mainly  of  ferromagnesian 
minerals,  we  can  illustrate  also,  in  a  very  rough  and 
general  way,  the  relation  between  them  and  their  meta- 
morphic  derivatives  in  the  following  table : 


Igneous  Rocks. 

Metamorphic  Rocks. 

Coarse-grained  feldspathic 
types,  such  as  granite,  etc..  . 

Gneiss. 

Fine-grained  feldspathic  types, 
such  as  felsite,  tuffs,  etc  

Slate  and  Schists. 

Ferromagnesian  rocks,  such  as 
dolerites  and  basalt  

Hornblende-,    Talc-, 
(etc  .  )  ,  Schists  and 

Serpentine. 

A  comparison  of  the  two  tables  will  show  that  gneisses 
and  schists  may  have  diverse  origins,  and  the  reason  for 
this  has  been  previously  pointed  out. 

Another  method  of  classification  which  has  been  recently  sug- 
gested is,  disregarding  the  origin  of  the  material  entirely,  to  consider 
only  its  chemical  composition.  According  to  this  the  metamorphic 
rocks  are  divided  into  groups.  The  earth's  crust  is  divided  vertically 
into  zones,  somewhat  as  described  above,  and  the  effect  of  the  meta- 
morphism  in  these  zones  upon  each  group  is  considered.  It  is  found 
that  material  of  a  given  composition  yields  rocks,  differing  in  mineral 
composition  and  texture,  according  to  the  zone  in  which  the  meta- 
morphism  occurred.  Thus  the  first  grouping  is  a  chemical  one,  while 
the  subdivisions  are  mineral  and  metamorphic,  and  in  this  way  the 
different  rocks  are  produced' and  classified. 

While  this  method  may  be  consistent  and  based  on  scientific 
principles,  it  is  not  a  practical  one  for  field  and  megascopic  use.  We 
cannot  make  analyses  of  rocks  under  ordinary  circumstances,  nor 
can  we,  in  most  cases,  even  estimate  megascopically  the  chemical 
composition  from  the  minerals  they  contain,  as  can  be  done  with 
the  microscope  and  thin  sections.  And  the  different  mineral  com- 
positions and  textures  pass  into  one  another  so  gradually,  that  only 


350  ROCKS  AND  ROCK  MINERALS 

in  a  very  general  way,  or  in  specific  cases,  can  we  say  whether  the 
rocks  have  been  metamorphosed  in  the  zone  of  fracture,  or  the  zone 
of  flowage. 

At  present  we  are  obliged,  for  practical  purposes  of  field 
work  and  megascopic  determination,  to  classify  quite  arbi- 
trarily the  metamorphic  rocks  according  to  their  evident 
mineral  composition  or  texture,  or  a  combination  of  both. 
Sometimes,  as  in  the  gneisses,  stress  is  laid  upon  the  first 
feature;  sometimes,  as  in  the  slates,  upon  the  second  one, 
in  accordance  with  whichever  one  is  the  most  evident  and 
characteristic. 

We  have  in  agreement  with  this  the  following  main 
groups  of  metamorphic  rocks. 

Grouping  of  Metamorphic  Rocks. 

1.  Gneisses  and  Feldspar  Rocks. 

2.  Mica-schist  and  Quartzite. 

3.  Slates  and  Phyllite. 

4.  Talc  and  Chlorite  Schists. 

5.  Hornblende  Schist. 

6.  Marble,  Lime  carbonate-silicate  Rocks. 

7.  Dolomite,  Magnesian  carbonate-silicate  Rocks. 

8.  Serpentine. 

9.  Iron  oxides  and  other  rocks. 

By  comparison  it  may  be  seen  that  the  above  is  in  the 
main  a  combination  of  the  two  tables  previously  given. 
The  more  important  of  the  members  are  given  in  italics. 


CHAPTER   XI. 

DESCRIPTION  OF  METAMORPHIC  ROCKS. 
GNEISS. 

THE  term  gneiss  is  not  only  the  name  of  a  particular 
kind  of  metamorphic  rock,  but  also,  in  a  wider  sense,  it  is 
used  as  an  expression  of  a  certain  texture.  Thus  when  we 
use  gneiss  as  a  name  in  the  limited  sense,  we  mean  a  rock 
which  has  the  composition  of  granite  —  quartz,  feldspar, 
and  mica  —  with  a  certain  foliated  texture;  if  we  say 
granite-gneiss,  syenite-gneiss,  diorite-gneiss,  we  use  it  in 
the  wider  sense,  and  denote  rocks  whose  composition  is 
indicated  by  the  first  word,  and  the  texture  by  the  second. 
The  only  general  definition  of  gneiss  which  will  cover  all 
cases  is,  that  they  are  metamorphic  rocks,  composed  of 
feldspar,  with  other  minerals,  which  have  a  certain  char- 
acteristic texture.  But,  as  everywhere  generally  used 
when  no  qualifier  is  prefixed,  common  gneiss,  which  is 
composed  of  quartz,  feldspar  and  mica,  as  stated  above, 
is  understood,  and  the  term  is  so  employed  in  this  book. 
If  the  wider  sense  is  meant  the  qualifier  is  given. 

Mineral  Composition.  Various  kinds  of  feldspar  are 
found  in  gneisses,  both  the  alkalic  and  soda-lime  varieties, 
but  they  can  rarely  be  distinguished  by  megascopic 
means.  The  mineral  is  white  to  gray  in  color,  or  reddish, 
as  in  granite,  and  is  apt  to  be  in  more  or  less  round,  or 
elongated,  lenticular,  formless  grains;  this  lack  of  definite 
form  makes  it  more  difficult  to  distinguish  from  the  quartz 
than  in  most  granites,  and  the  cleavage  should  be  carefully 
sought.  Sometimes  large  grains,  the  size  of  a  pea,  or  even 
larger,  occur,  giving  the  gneiss  a  porphyritic  character;  if 
the  cleavages  of  these  are  examined  against  the  light,  it 

351 


352  ROCKS  AND   ROCK  MINERALS 

may  be  often  observed  that  they  are  Carlsbad  twins. 
Such  large  crystals  may  indeed  have  been  the  phenocrysts 
of  a  former  porphyritic  granite,  or  they  may  have  been 
feldspar  pebbles  of  a  conglomerate  or  arkose,  or  they  may 
have  been  made  by  injected  material. 

The  quartz  is  also  in  more  or  less  round  grains  or  lentic- 
ular masses,  or  in  granular  aggregates  with  the  feldspar. 
Its  color  is  white  or  gray,  sometimes  reddish,  rarely  bluish. 
In  the  larger  grains  it  is  easily  recognized  by  its  greasy 
luster  and  conchoidal  fracture. 

The  mica  may  be  either  biotite  or  muscovite,  or  a  mix- 
ture of  both.  The  biotite  is  black  or  dark  brown,  the 
muscovite  is  white  or  yellowish  to  light  brown,  sometimes 
pale  green.  The  mineral  does  not  have  any  distinct  crystal 
form,  but  is  in  flakes,  shreds  or  irregular  leaves,  drawn  out 
in  bands,  or  in  thin  patches.  It  usually  lies  stretcher1 
out  along  the  structure  planes  of  the  rock,  and  in  large 
part  its  easy  cleavage,  thus  arranged  in  one  direction,  con- 
ditions the  schistosity  or  cleavage,  and  gives  emphasis  to 
the  gneissoid  texture.  Thus  the  surface  of  chief  fracture 
of  a  flake  of  the  rock  may  appear  to  be  largely  coated 
with  mica,  and,  judging  from  this  alone,  one  would  be  apt 
to  gain  an  exaggerated  idea  of  the  relative  amount  of  it  in 
the  rock;  the  surface  of  cross  fracture  should  also  be 
examined  to  gauge  correctly  its  relative  amount,  as  com- 
pared with  the  other  mineral  constituents.  This  is  also 
especially  true,  in  the  mica  schists,  and  in  those  gneisses, 
which,  by  decrease  of  feldspar  and  increase  in  mica,  form 
transitions  into  these  latter  rocks.  This  effect  is  also 
more  marked  in  many  gneisses,  because  there  is  a  tendency 
for  the  quartz  and  feldspar  to  be  collected  in  layers,  which 
alternate  with  layers  of  mica. 

Hornblende  may  occur  in  gneisses,  sometimes  associated  with  the 
mica,  sometimes  alone,  forming  a  special  variety.  It  is  seen  in  dark, 
prismatic  crystals  without  good  terminations,  as  in  granite,  syenite, 
etc.  Minute  crystals  may  be  aggregated  into  flattened  lumps  and 
layers. 


PLATE  35. 


A.   GARNET   IN   GNEISS,    WITH   ZONE 
OF   GROWTH. 


B.   GNEISSOID   CONTORTED   SCHIST. 
(U.  S.  Geological  Survey.) 


DESCRIPTION  OF  METAMORPHIC  ROCKS         353 

Besides  these,  many  other  minerals  may  occur  in  gneisses, 
sometimes  so  prominently  as  to  form  special  varieties.  Of  these 
garnet,  of  a  dark  red  common  variety,  is  perhaps  the  most  con- 
spicuous. The  crystals  are  sometimes  large,  as  compared  with  the 
size  of  the  other  rock  constituents.  Epidote  may  also  be  discovered, 
as  well  as  graphite,  in  some  varieties.  Sillimanite,  a  mineral  with  the 
same  composition  as  andalusite  and  cyanite,  is  sometimes  seen  in 
gneiss,  in  bundles  and  brush-like  groups  of  slender  fibers  or  prisms. 
Tourmaline  occurs  also  under  circumstances  similar  to  those  which 
obtain  in  granite.  In  some  gneisses  the  mica  may  be  partly,  or 
wholly,  replaced  by  chlorite,  usually  from  alteration. 

Texture.  This  has  been  already  described  in  large,  part 
under  the  general  remarks  on  metamorphic  rocks  and  what 
has  been  said  above  respecting  the  mica.  The  essence 
of  the  texture  consists  in  the  layers  of  mingled  quartz 
and  feldspar,  which  are  separated  by  drawn  out  layers  of 
mica.  Where  the  amount  of  mica  is  small,  the  gneissoid 
texture  is  less  evident,  and  it  increases  with  the  increase 
of  mica.  Sometimes  these  layers  are  thick  and  coarse, 
giving  a  pronounced  gneissoid  effect,  sometimes  the  layers 
are  extremely  thin.  In  some  cases  the  layers  continue 
their  individual  character  for  considerable  distances,  in 
others  they  are  very  short,  lenticular,  and  are  closely 
interlaminated.  According  to  these  appearances,  dif- 
ferent varieties  of  gneiss  have  been  named  on  a  textural 
basis.  The  gneissoid  texture  is  sometimes  scarcely  per- 
ceptible in  a  hand  specimen,  but  clearly  seen  on  a  large, 
exposed  surface  of  the  rock.  This  is  especially  the  case  in 
rocks  which  were  originally  granites,  but  which,  by  pres- 
sure and  shearing,  have  been  converted  into  gneiss. 

The  texture  described  above,  the  banding  or  schistosity, 
may  extend  for  long  distances  in  straight,  regular  lines, 
or  it  may  be  curved,  folded,  contorted,  or  faulted,  often  in 
the  most  complex  and  remarkable  manner,  and  on  any 
scale,  even  to  a  very  minute  or  even  microscopic  one.  Ex- 
amples of  such  intricately  folded  and  compressed  gneisses 
are  seen  on  Plates  35  and  36.  Such  folding  testifies  ia 
general  to  repeated  dynamic  movements,  with  shearing 


354  ROCKS  AND  ROCK  MINERALS 

and  folding,  the  earlier  ones  producing  the  gneissoid 
structure  and  the  later  ones  crumpling  it  up,  though  it  is 
possible  that  in  some  cases  the  two  things  are  simultaneous, 
In  some  gneisses,  as  in  some  granites,  a  definite  por~ 
phyritic  texture  may  be  present,  with  large  and  definite 
crystals  of  feldspar,  which  show  more  or  less  distinct 
crystal  form. 

Such  gneisses  are  to  be  generally  regarded  as  originally  porphyritic 
granites,  which  have  had  the  gneissoid  texture  imposed  upon  them, 
though  in  some  cases,  it  may  be,  that  the  large  crystals  have  been 
formed  in  gneiss  of  a  different  origin  by  growth  from  injected  material. 
Such  gneisses  are  allied  to,  and  may  pass  over  into,  types,  which,  with 
a  short,  thick,  lenticular  texture,  contain  ovoid  masses  of  feldspar  or 
quartz.  The  ovoid  bodies  are  called  "  eyes  "  (German,  augen}, 
and  the  rocks  containing  them  "  augen-gneiss  '*  from  the  German 
name,  or  "  eyed-gneiss."  As  explained  on  a  previous  page  on  the 
texture  of  metamorphic  rocks,  they  may  be  of  quite  diverse  origin. 

In  some  gneisses  are  to  be  seen  pebbles  of  various  kinds 
of  previously  existent  rock  masses,  of  granite,  quartzite, 
etc.  They  are  apt  to  be  drawn  out  into  flattened  lenticu- 
lar masses,  but  their  original  character  is  evident,  and  it  is 
clear  that  the  gneiss  in  such  a  case  was  originally  a  con- 
glomerate, whose  finer  material  has  been  metamorphosed, 
leaving  the  larger  pebbles  mostly  unchanged,  save  in 
shape. 

Color.  The  color  of  gneisses  is  too  variable  a  feature  to 
be  of  any  value  as  a  special  character.  It  depends  on  the 
color  of  the  quartz  and  feldspar,  and  on  the  relation  of  these 
to  the  amount  of  biotite,  or  other  dark  colored  minerals, 
they  may  contain.  Also,  in  gneisses  of  sedimentary  origin, 
carbonaceous  material  may  be  present  and  in  the  form  of 
graphitic  material  color  the  rock  very  dark.  Hence  we 
find  them  from  almost  white  passing  through  light  shades 
of  red  or  gray  into  darker  ones,  into  brown  and  green, 
and  even  black. 

Chemical  Composition.  As  the  sources  of  the  material 
from  which  gneisses  have  been  made  are  varied,  so  do  we 


DESCRIPTION  OF  METAMORPHIC  ROCKS         355 


find  great  variability  in  their  chemical  composition,  BO 
much  so  that  this  character  cannot  be  relied  upon  as 
having  any  special  value  as  one  of  their  definite  features. 
Since  they  are  composed  of  quartz  and  feldspar  in 
notable  amount  they  must  contain  silica,  alumina  and 
alkalies,  and  they  usually  have  also  more  or  less  iron  and 
lime,  but  these  oxides  may  vary  within  wide  bounds,  as 
may  be  seen  from  the  following  table  of  analyses  of  a 
few  typical  gneisses. 


Si02 

A12O3 

Fe2O3 

FeO 

MgO 

CaO 

NasO 

K2O 

H2O 

XyO 

Total 

I.. 

TT 

71.0 
65.1 

15.0 
16.4 

1.1 
0.9 

1.8 

5  6 

0.7 
2.4 

0.3 

?,  4 

2.5 

3  3 

5.8 
1  9 

1.1 

0  7 

0.5 
1.3 

99.8 
100.0 

III. 
IV. 
V 

78.3 
82.4 
52.2 

10.0- 

11.3 

18.8 

1.8 
1.0 

2.7 

1.8 
0.3 

5  3 

1.0 
0.2 
5.1 

1.7 
0.2 
8  0 

2.7 
0.6 
3  3 

1.3 
1.0 
1  6 

1.0 
2.5 
1  4 

0.9 
0.2 

1  7 

100.5 
99.7 
100  1 

VT 

44.5 

17.5 

3.4 

191  fi 

5.7 

3  3 

3  fi 

3  R 

1  6 

5.2 

100  9 

I,  Granite-gneiss,  Lincoln,  Vermont;  II,  Garnet-biotite-gneiss, 
Fort  Ann,  Washington  Co.,  N.  Y. ;  III,  Gneiss,  fine  grained,  Great 
Falls  of  Potomac  River,  Md. ;  IV,  Schistose  gneiss,  Marquette 
region,  Michigan.  V,  Plagioclase-Gneiss,  Mokelumne  River,  Cali- 
fornia; VI,  Gneiss  (Kinzigite),  Schenkenzell,  Black  Forest,  Baden. 

On  the  other  hand,  as  stated  in  the  introduction  to 
metamorphic  rocks,  the  analyses  may  suggest  a  clue  to 
the  origin  of  the  material.  Thus  analyses  III,  IV,  and  VI 
above  are  quite  unlike  those  of  any  igneous  rock,  and  are 
almost  certainly  of  material  of  sedimentary  origin,  while 
the  others  may  be  of  igneous  derivation. 

Varieties  of  Gneiss.  A  very  great  number  of  varieties 
of  gneiss  have  been  distinguished  by  geologists  and 
petrographers.  These  have  been  based,  partly  on  dif- 
ferences in  texture,  such  as  "  banded-gneiss,"  "  lenticular- 
gneiss,"  "  augen-gneiss,"  etc.,  partly  on  the  presence  of 
some  characteristic  or  unusual  mineral,  such  as  "  biotite- 
gneiss,"  "  hornblende-gneiss/'  "  epidote-gneiss,"  etc.,  and 


356  ROCKS  AND   ROCK  MINERALS 

partly  on  general  composition,  such  as  "  granite-gneiss," 
"  diorite-gneiss,"  etc.  In  the  latter  case  the  term,  as 
explained  in  the  introductory  paragraph,  is  used  in  the 
sense  of  a  general  textural  modifier.  It  would  not  be 
suitable  in  a  work  of  this  kind  to  give  a  description  of  all 
these  varieties,  but  a  few  of  the  most  prominent  may  be 
mentioned.  The  textural  modifications  have  been  already 
sufficiently  considered  under  the  heading  of  texture. 

Of  mineralogic  varieties,  by  common  gneiss,  or  "  gneiss  "  for  short, 
mica-gneiss  is  meant.  If  further  distinction  is  required,  the  kind  of 
mica  present  may  be  stated,  and  we  thus  have  biotite-gneiss,  mus- 
covite-gneiss,  or  biotite-muscovite-gneiss.  If  the  mica  is  accompanied 
or  replaced  by  some  prominent  mineral,  as  is  often  the  case,  other 
varieties  are  formed,  such  as  hornblende-gneiss,  epidote-gneiss,  tour- 
maline-gneiss, garnet-biotite-gneiss,  etc.  The  different  prominent 
minerals,  which  may  thus  take  part,  have  been  already  described 
under  composition.  Of  the  varieties  based  on  general  composition, 
it  may  be  said  that  all  of  the  different  varieties  of  coarser-grained, 
feldspathic,  igneous  rocks  may  occur  with  pronounced  gneissoid 
texture.  In  accordance  with  this  we  have  granite-gneiss  —  by  far 
the  most  common  variety  —  syenite-gneiss,  diorite-gneiss,  and  even 
gabbro-  and  anorthosite-gneisses.  Sometimes  this  texture  has  been 
imposed  upon  the  igneous  rocks  after  they  had  solidified,  by  intense 
pressure  and  shearing,  and  sometimes  while  they  were  still  soft,  pasty 
and  crystallizing,  by  forced  differential  flowage,  due  to  various  causes. 

Inclusions  in  Gneiss.  It  is  very  common  to  find 
inclusions,  or  smaller  rock  masses,  embedded  in  gneiss, 
which  differ  in  a  marked  degree  in  mineral  composition, 
texture,  etc.,  from  the  main  rock  body  which  encloses 
them.  Thus  lenticular  masses  of  quartz  frequently  occur, 
and  of  very  variable  size.  They  may  be  the  remains  of  a 
quartz  pebble  of  a  conglomerate,  as  explained  under 
texture,  or  they  may  have  been  deposited  from  solution 
in  some  lenticular  cavity,  opened  in  the  folding  of  the  rock 
masses.  This  case  may  sometimes  be  detected,  in  that 
the  quartz  mass  tends  to  possess  a  comb  structure,  being 
composed  of  an  aggregate  of  crystals  whose  prism  direc- 
tions are  set  perpendicular  to  the  wall  of  the  cavity. 


PLATE   36. 


DESCRIPTION  OF  METAMORPHIC  ROCKS         357 

In  many  gneisses  irregular  spots,  streaks,  and  lines  of 
pegmatite  occur,  similar  to  those  in  granite.  In  addition 
to  the  quartz,  feldspar  and  mica,  they  often  contain  the 
accessory  minerals  seen  in  granite-pegmatite  dikes,  such 
as  tourmaline,  apatite,  beryl,  garnet,  topaz,  etc.  In  the 
latter  case  they  probably  represent  the  remains  of  former 
granite-pegmatite  dikes,  which  have  been  folded  up  or 
squeezed  out  in  dynamic  metamorphic  processes,  but 
not  all  of  the  pegmatitic  modifications  seen  in  gneiss  are 
to  be  certainly  ascribed  to  such  an  origin,  for  they  may 
have  been  produced  by  secretions  from  later  solutions  of 
heated  waters  moving  through  the  rock  mass.  The 
beautiful  crystals  of  orthoclase,  of  the  varieties  called 
adular  and  moonstone,  occurring  in  some  gneisses,  have 
been  probably  produced  in  this  way. 

Also  there  are  frequently  seen  in  gneiss,  spots,  streaks 
and  irregularly  curved  and  winding  ribbons  of  white  or 
pink  felsite,  or  fine-grained  granite  similar  to  the  aplite  of 
granites.  These  may  be  former  aplite  dikes  folded  up, 
or  later  granitic  intrusions  or  secretions  from  heated  solu- 
tions. They  are  sometimes  seen  in  the  most  complicated 
systems  of  network  passing  through  the  rock,  and 
they  may  not  have  any  definite  wall  against  the  gneiss, 
as  is  the  case  with  regular  aplite  dikes.  By  their  fold- 
ings, faultings,  and  contortions,  they  often  show  very 
clearly  the  movements  which  the  general  rock  body  has 
undergone. 

Included  masses  of  other  kinds  are  also  frequently  met 
with  in  gneiss.  Thus  the  streaks  and  smears,  produced  by 
aggregates  of  the  dark-colored  or  ferromagnesian  minerals, 
such  as  areseen  in  granites  and  are  described  as  "schlieren," 
are  found  in  gneiss,  and  may  have  a  similar  origin.  Also, 
irregular  masses,  strips,  and  lenticular  bodies  of  other 
schists  occur,  which,  if  the  gneiss  has  been  derived  from  a 
former  mass  of  igneous  rock,  may  have  been  included  or 
enveloped  fragments  of  the  stratified  beds,  into  which  it 
was  intruded. 


358  ROCKS  AND  ROCK  MINERALS 

While  the  study  of  thin  sections  under  the  microscope  is  often  of 
great  assistance  to  the  field  study  of  a  gneiss,  in  the  endeavor  to 
ascertain  its  origin  and  to  thus  understand  better  its  relation  to  other 
rocks,  it  is  by  no  means  always  necessary.  Very  much  may  be  done 
by  careful  observation  in  the  field  of  all  the  facts  ascertainable,  and 
by  the  thoughtful  correlation  of  these  facts  with  one  another. 
From  place  to  place  the  rock  should  be  minutely  studied  with  the 
lens  and  any  change  in  mineralogical  composition  or  texture  noted. 
The  following  embody  some  of  the  chief  points  which  should  be  looked 
for,  to  distinguish  rocks  originally  igneous  from  those  of  sedimentary 
origin.  The  igneous  ones  are  more  apt  to  have  a  uniform  composi- 
tion and  texture  over  large  areas.  The  region  of  the  contact  with 
other  rocks  should  be  carefully  observed,  to  see  if  there  are  any 
remains  of  a  former  endomorphic  contact  visible,  such  as  a  diminish- 
ing of  grain,  or  the  assumption  of  a  porphyritic  texture,  as  well  as 
the  appearance  of  pneumatolytic  minerals,  of  which  tourmaline 
may  be  cited  as  a  specially  important  example.  The  remains  of 
former  aplite  dikes  and  pegmatite  veins,  as  described  above,  should 
also  be  noted  in  this  connection.  The  enveloping  or  bordering  rocks 
should  be  carefully  studied  to  see  if,  by  change  in  mineral  composition, 
in  texture,  and  in  the  presence  of  tourmaline,  or  other  pneumato- 
lytic minerals,  any  remains  of  a  former  aureole  of  contact  metamor- 
phism  may  be  discovered.  The  character  of  the  plane  of  contact 
of  the  gneiss  and  its  neighboring  rocks  should  be  examined  to  see, 
if  possible,  whether  they  are  interwoven,  as  contorted  interlaminated 
beds  might  be  expected  to  be,  or  whether  the  gneiss  cuts  directly 
across  them.  In  fact  all  of  the  field  characters  indicative  of  intru- 
sion, which  have  been  described  under  granite,  should  be  looked 
for,  under  the  veil  which  metamorphism  has  cast  over  the  region 
under  study.  They  may,  of  course,  have  been  entirely  obliterated, 
but  some  of  them  may  persist  and  be  valuable  indicators. 

In  sedimentary  gneisses,  on  the  other  hand,  more  rapid  changes, 
from  place  to  place,  in  composition  and  texture  may  be  looked  for, 
both  on  a  large  and  on  a  minute  scale.  The  remains  of  former 
pebbles,  or  small,  lenticular  masses  of  different  composition  indicative 
of  them,  should  be  sought  for.  The  presence  of  carbonaceous  matter, 
or  graphite,  diffused  through  the  rock,  or  collected  in  spots  or  streaks, 
is  also  of  use  in  indicating  this  origin.  The  absence  of  any  of  the 
signs  of  intrusion,  and  the  character  of  the  contact,  as  mentioned 
above,  may  also  be  of  value  in  this  connection.  Not  too  much  stress 
must  be  placed  on  the  mere  presence  of  felsitic  and  pegmatitic 
veins  or  dikes,  as  these  may  have  been  injected  into  sedimentary 
rocks,  as  well  as  into  igneous  ones.  Their  character,  number,  dis- 
position, and  contact  wall  must  also  all  be  considered  in  relation  to 
the  rock  mass  they  accompany. 


DESCRIPTION  OF  METAMORPHIC  ROCKS         359 

If,  to  the  facts  observed  in  the  field,  a  chemical  analysis  of  a  well 
selected  specimen,  or  series  of  specimens,  of  the  gneiss  can  be  added, 
this  may  prove  in  addition  of  great  value.  This  has  been  commented 
on  elsewhere  and  need  not  be  repeated. 

When  all  is  said  and  done,  however,  it  must  always  be  remem- 
bered, as  Rosenbusch,  the  great  German  petrologist,  has  well  said, 
"  there  is  no  formula  by  which  the  derivation  of  a  gneiss  may 
be  invariably  determined."  It  must  not  be  done  on  any  one 
character  alone,  but  all  must  be  taken  into  account  and  relatively 
balanced,  and  even  when  this  is  done,  it  is  impossible  in  many  cases 
to  say  if  the  gneiss  has  been  derived  directly  from  an  igneous  rock, 
or  whether  the  material  of  the  latter  may  not  have  passed  through 
an  intermediate  sedimentary  stage. 

General  Properties  and  Uses  of  Gneiss.  Those  gneisses, 
which  under  the  action  of  metamorphic  agencies  have 
been  thoroughly  recrystallized,  form  solid  and  massive 
rocks,  whose  general  properties  closely  resemble  the 
massive  igneous  ones.  Thus  granitic  gneiss  closely  resem- 
bles granite,  and  is  used  in  the  same  manner  for  building 
and  structural  purposes.  But  often  gneiss  contains  so 
much  mica,  that  it  has  too  easy  a  cleavage  to  be  of  much 
value.  In  general  a  gneiss  should  be  so  placed,  that  the 
plane  of  chief  fracture  lies  in  the  mortar  bed  with  the  cross 
fracture  exposed;  otherwise  it  is  liable,  like  some  sedi- 
mentary rocks,  to  split  and  scale  badly.  Those  gneisses 
which  have  assumed  their  texture  under  conditions  of  dry 
crushing  and  shearing  are  very  tender  and  friable  rocks, 
which  fall  to  pieces  readily  under  the  blow  of  the  hammer, 
and  are  of  little  value.  The  granite-gneiss  of  portions  of 
the  Alps,  and  the  anorthosite-gneiss  of  parts  of  the  Adi- 
rondacks,  are  examples  of  this.  The  jointing,  erosion 
forms,  etc.,  of  granite-gneiss  are  similar,  in  general,  to  what 
is  stated  under  granite.  So  also  is  the  weathering,  and 
gneisses  form  fertile  sandy  soils,  which  pass  into  loamy 
ones,  as  the  decay  of  the  feldspar  and  its  alteration  into 
kaolin  becomes  more  complete. 

Occurrence  of  Gneiss.  Gneiss,  especially  common  or 
mica-gneiss,  is  one  of  the  most  common  and  widely  dis- 


360  ROCKS  AND  ROCK  MINERALS 

tributed  of  rocks.  The  occurrence  of  the  metamorphic 
rocks  in  general  has  been  already  commented  on,  and  it 
was  stated  that  they  are  found  in  mountain  regions  and 
in  those  areas  where  the  sedimentary  beds  have  been 
eroded,  as  a  basement  upon  which  these  later  rocks  rest. 
In  such  places  common  gneiss  is  usually  the  most  promi- 
nent rock.  .  Owing  to  this,  it  is  spoken  of  by  many  geolo- 
gists as  "  the  basal  gneiss,"  or  "  fundamental  gneiss,"  and 
as,  in  many  places,  it  is  clearly  the  oldest  rock  of  which  we 
have  any  knowledge,  some  believe  that  they  see  in  it  the 
primitive  crust  of  the  earth.  The  Archaean,  as  it  is  now 
used  as  a  division  of  geologic  time,  is  almost  entirely  com- 
posed of  gneiss,  and  to  attempt  to  mention  all  the  localities 
of  the  rock,  would  be  practically  equivalent  to  a  descrip- 
tion of  the  occurrence  of  the  Archaean.  Gneisses  are  not 
of  course  restricted  to  the  Archaean;  they  occur  in  later 
formations,  into  the  Mesozoic.  Gneisses  are  found  all 
over  New  England,  and  southward  along  the  Piedmont 
plateau  into  Georgia;  in  the  Adirondacks;  in  the  Rocky 
Mountains'  region,  the  Sierra,  and  other  places  in  the 
United  States;  they  cover  large  parts  of  eastern  Canada 
and  are  prominent  in  Scotland,  Norway  and  Sweden, 
Finland,  parts  of  Germany,  and  in  the  Alps.  In  all  of  these 
regions  different  varieties,  such  as  hornblende-gneiss, 
occur  associated  with  the  common  kind. 

Granulite.  Associated  with  gneisses  in  a  number  of  localities  is  a 
schistose,  to  thin  schistose,  rock  composed  almost  wholly  of  quartz 
and  feldspar.  It  is  nearly,  or  wholly,  free  from  mica,  and  is  usually 
of  fine  to  dense  grain,  so  that,  except  for  its  schistose  character  and 
place  of  occurrence,  it  is  much  like  an  igneous  felsite  or  aplite.  It  is 
apt  to  carry  minute  red  garnets,  and  sometimes  small  quantities  of 
other  minerals,  such  as  cyanite,  tourmaline,  or  hornblende,  can  be 
detected  with  the  lens.  Chemically,  it  is  similar  in  composition  to 
somefelsites  orthe  aplite  varietyof  granite,  and  it  probably  represents 
in  general  former  igneous  rock  of  this  nature  which  has  been  in- 
volved in  the  metamorphic  processes.  Such  granulites  occur  in 
Saxony  and  other  places  in  Germany,  where  they  were  first  studied  ; 
in  Sweden,  Finland,  Austria,  etc.,  in  Europe;  in  New  England  and  in 
the  Adirondack  region  of  New  York. 


DESCRIPTION  OF  METAMORPHIC  ROCKS         361 


MICA-SCHIST. 

Mica-schist  is  a  rock  which  is  closely  related  on  the  one 
hand  to  gneiss,  and  on  the  other  to  quartzite.  It  is  not 
only  a  very  common  companion  of  gneisses,  in  regions  of 
metamorphic  rocks,  but  in  many  places  gneiss  grades  into 
mica-schist,  so  that  no  definite  line  can  be  drawn  between 
them.  It  has  also  many  other  analogies  with  gneiss,  some 
of  which  will  be  presently  mentioned.  Of  that  great  class 
of  rocks  known  as  schists,  it  is,  excluding  gneiss,  if  the 
latter  be  reckoned  among  them,  the  most  widely  dis- 
tributed and  important. 

Composition  —  Minerals  and  Texture.  The  essential 
minerals  of  mica-schist  are  quartz  and  mica,  and  it  is  es- 
pecially the  latter  which  gives  the  rock  its  particular  char- 
acter. Different  varieties  of  mica  occur;  the  most  common 
is  a  silvery  white  muscovite;  biotite  of  a  dark  color  is  com- 
mon, while  the  soda-bearing  mica  —  paragonite  —  is  rare. 
Muscovite  and  biotite  occur  alone,  and  also  in  combination, 
as  in  gneiss.  The  micas  are  in  irregular  leaves  or  tablets, 
without  crystal  boundaries,  or  in  leafy  or  foliated  aggre- 
gates; biotite  and  muscovite  are  found  intergrown,  and 
often  so  that  they  have  a  common  cleavage.  The  micas 
lie  with  their  cleavage  planes  in  the  direction  of  schistosity, 
and  it  is  this  which  produces  the  extraordinary  fissile 
character  of  the  rock.  They  are  also  very  often  curved, 
bent,  or  twisted,  as  may  be  easily  seen  by  the  reflections 
from  their  cleavage  surface.  The  cleavage  of  the  mica  is 
so  marked  that  the  surface  of  chief  fracture,  or  the  schistose 
plane  of  the  rock,  appears  completely  coated  by  it,  and  it 
may  produce  the  impression  that  it  is  the  only  mineral 
present;  to  see  the  quartz,  the  other  essential  component, 
the  cross  fracture  should  be  examined  with  the  lens.  The 
quartz  forms  irregular  grains,  or  aggregates  of  grains, 
and  these  are  sometimes  arranged  in  small  lenses,  and 
sometimes  in  thin  layers,  concordant  with  the  layers  of 
mica. 


362  ROCKS  AND  ROCK  MINERALS 

Mica-schists,  while  they  are  very  often  composed  of 
these  two  minerals  alone,  also  very  commonly  carry  crys- 
tals, often  of  large  size,  of  other  minerals.  The  most 
common  of  these  is  a  dark  red  garnet,  sometimes  sparsely, 
but  generally  thickly,  sprinkled  through  the  rock,  and 
varying  in  size  from  that  of  coarse  shot  to  that  of  a  plum. 
These  garnets  are  often  in  the  form  of  simple,  rounded 
nodules,  but  in  most  cases  they  show  more  or  less  distinct 
crystal  form,  and  sometimes  they  are  beautifully  crystal- 
lized in  the  shapes  mentioned  in  the  description  of  this 
mineral.  This  garnetiferous  variety  of  mica-schist  is  a 
very  common  metamorphic  rock;  in  New  England  it  is 
widely  distributed  among  the  bowlders  of  the  glacial  drift. 

Other  minerals  which  occur  in  mica-schist,  in  a  manner 
similar  to  garnet,  are  staurolite,  often  with  garnet,  cyanite, 
epidote,  andalusite,  and  hornblende.  These  sometimes 
are  in  large  and  well-formed  crystals,  which,  especially 
staurolite,  andalusite,  and  cyanite,  are  not  infrequently 
colored  dark,  by  included  carbonaceous  matter.  Graphite 
occurs  in  some  mica-schists  in  quantity  sufficient  to  pro- 
duce a  distinct  variety.  Graphite  is  such  a  strong  coloring 
matter,  that  a  relatively  small  amount  will  cause  the  rock 
to  appear  as  if  almost  entirely  composed  of  it;  in  conse- 
quence unsuccessful  attempts  have  been  made  in  places 
to  exploit  such  schists  for  graphite. 

Hornblende,  when  it  occurs,  is  in  dark-colored  prisms; 
by  its  increase  in  amount  transitions  into  amphibolite  or 
hornblende  schist  are  formed. 

Cyanite,  andalusite,  and  staurolite  occur  in  prismatic 
crystals,  which  may  attain  a  length  of  several  inches. 
Their  formation  is  contemporaneous  with  the  metamor- 
phism  of  the  rock,  and  they  produce  a  pseudo-porphyritic 
texture  as  previously  explained  on  page  342.  Another 
variety  of  mica-schist  is  one  which  contains  more  or  less 
calcite  mingled  with  the  quartz;  it  is  readily  detected  by 
its  effervescence  with  acids.  This  variety  is  especially 
apt  to  contain  accessory  garnet,  epidote,  hornblende,  etc. 


DESCRIPTION  OF  METAMORPHIC  ROCKS 


363 


The  parallel  texture  of  the  rock  is  its  especial  feature, 
and  its  ready  fissility  is  produced  by  the  mica.  If  the 
components  are  in  thin,  parallel  layers,  the  surface  of  rock 
cleavage  is  smooth  and  flat;  if  the  lenticular  arrangement 
of  the  quartz  is  prominent,  the  surface  is  uneven  or  lumpy. 
Frequently  the  surfaces  of  schistosity  are  bent,  folded 
and  crumpled,  showing  pressures  and  shearing  secondary 
to  its  production. 

Chemical  Composition.  As  in  the  gneisses,  the  chemical 
composition  of  these  rocks  is  too  variable  a  feature  to  be 
of  specific  value.  This  comes  from  the  natural  variability 
in  the  composition  of  the  sediments  from  which  they  are 
formed.  In  addition,  not  many  of  these  rocks  have  been 
chemically  investigated,  and  some  of  the  older  analyses 
have  been  very  poorly  executed.  It  is  clear,  however, 
that  they  must  contain  silica,  alumina,  and  potash,  to 
form  the  quartz  and  mica,  and  also  magnesia  and  iron,  if 
biotite  is  present.  The  excess  of  magnesia  over  lime, 
taken  with  the  high  silica,  is  a  character  foreign  to  igneous 
rocks,  and  is  clearly  indicative  of  sedimentary  origin. 
They  are  probably  formed  mostly  by  the  metamorphism 
of  feldspathic  sandstones.  Two  analyses  of  typical  sam- 
ples carried  out  in  the  laboratory  of  the  United  States 
Geological  Survey  are  here  appended. 


Si02 

A12O3 

Fe2O3 

FeO 

MgO 

CaO 

NaaO 

K20 

H2O 

XyO 

H20 

64.7 
64.8 

16.4 
14.4 

1.8 
1.8 

3.8 
4.5 

3.0 

2.3 

0.1 
2.3 

0.1 

1.4 

5.6 
5.0 

3.1 
2.0 

0.8 
1.9 

99.4 
100.4 

General  Properties.  The  color  of  these  rocks  varies  from 
very  light,  through  gray,  yellow,  or  brown  tones,  into  very 
dark,  depending  on  the  proportions  of  light  and  dark  mica, 
the  presence  of  carbonaceous  material,  and  in  part  on  the 
amount  of  alteration  of  the  iron-bearing  biotite.  Some 
pure  muscovite  schists  are  almost  silvery  white  or  light 


364  ROCKS  AND  ROCK  MINERALS 

gray.  The  hardness  and  firmness  of  the  rock  depend  on 
the  proportion  of  mica;  the  more  this  is  present,  the  softer 
and  more  easily  cleavable  it  is.  For  this  reason  they  are 
of  little  or  no  value  for  practical  purposes.  Inclusions  of 
various  kinds  occur  in  mica-schists  as  in  gneiss,  thus  veins 
and  lenticular  masses  of  quartz,  deposited  from  solution 
in  cracks  and  cavities  opened  by  movement  and  foldings 
of  the  rocks,  are  common.  They  also  contain  in  places 
lenticular  masses  of  other  schists,  which  may  vary  from 
very  small  to  huge  dimensions.  And  sometimes  they  are 
penetrated  by  seams  and  patches  of  granite,  felsite,  and 
pegmatite  as  the  result  of  granitic  injections.  With 
respect  to  the  alteration  of  mica-schist,  the  varieties  com- 
posed of  muscovite  are  chiefly  mechanically  disintegrated 
by  the  action  of  weathering  without  much  chemical  change. 
The  muscovite  resists  alteration  energetically,  and  the 
gravelly  or  sandy  soils  formed,  are  in  consequence  filled 
with  its  sparkling  flakes.  Where  much  biotite  is  present  it 
alters  easily;  the  rocks  turn  yellow  or  brown,  lose  their 
luster,  and  eventually  much  limonite  is  separated  out. 

Varieties  and  Occurrence.  The  varieties  composed 
chiefly  of  muscovite,  or  with  associated  garnet,  are  the 
most  usual  kinds,  and  are  found  all  over  the  world  as 
common  rocks  in  metamorphic  regions,  and  are  generally 
associated  with  gneisses.  They  cover  large  areas  in  New 
England  and  extend  southward  to  Georgia.  Biotitic 
varieties  are  also  very  commonly  found  with  them. 
Staurolitic  mica-schist  occurs  in  many  places  in  New 
England,  and  in  Maryland,  and  elsewhere  along  the  Pied- 
mont plateau;  it  is  found  in  Scotland  and  various  localities 
in  Europe,  in  Brazil  and  elsewhere.  Cyanite-mica-schist 
occurs  in  various  places  in  New  England;  a  variety  in 
which  the  mica  is  paragonite  comes  from  the  St.  Gothard 
region  in  the  Alps,  and  is  seen  in  mineral  collections  on 
account  of  the  beautiful  crystals  of  cyanite  it  contains; 
the  common  kind  with  muscovite  is  found  in  many  places. 
Hornblendic  mica-schists  occur  as  included  lenticular 


DESCRIPTION  OF  METAMORPHIC  ROCKS         365 

masses,  often  of  large  dimensions,  in  various  places,  in  the 
ordinary  mica-schists.  Graphitic  mica-schist  is  found  in 
Connecticut  and  other  places  in  New  England,  various 
localities  in  Germany,  Norway,  etc.  Andalusite-mica- 
schist  occurs  in  the  White  Mountains  in  New  England, 
in  Scotland,  Spain,  Germany,  etc. 

An  interesting  variety  is  the  conglomerate-mica-schist,  in  which  the 
rock  contains  pebbles  of  quartz,  granite,  and  other  rocks  which  are  very 
apt  to  be  flattened,  lenticular,  or  drawn  out  by  pressure  and  shearing. 
It  is  closely  related  to  the  conglomerate-gneiss  previously  described 
and  has  had  a  similar  origin.  Such  rocks  occur  in  Massachusetts,  in 
Vermont,  Scotland,  Sweden,  etc. 

Transitions  and  Relation  to  other  Rocks.  The  gneisses 
formed  from  sediments  and  the  mica-schists  have  both 
been  made  from  similar  rocks;  from  feldspathic  sand- 
stones, shales  and  conglomerates.  In  the  mica-schists  the 
feldspar  has  been  converted  into  mica;  in  the  gneisses  it 
has  mostly  persisted  or  been  recrystallized.  It  is  not  in- 
tended in  this  statement  to  affirm  that  this  is  the  only 
origin  for  mica-schists,  only  the  most  usual  one;  they  may 
have  been  formed  in  some  cases  from  quartzose-f  eldspathic 
igneous  rocks,  though  positive  evidence  on  this  point  is 
wanting.  In  this  connection  what  is  said  elsewhere  of 
phyllites  should  be  consulted.  On  the  whole  it  would 
seem  most  probable  that  the  gneisses  have  been  formed 
most  often  from  the  conglomerates  and  coarser-grained 
sandstones,  the  mica-schists  from  the  finer-grained  ones, 
and  from  the  shales,  though  many  exceptions  must  occur. 

It  is  therefore  easy  to  understand  that  many  mica-schists  contain 
more  or  less  of  feldspar  grains  among  those  of  quartz,  which  are  diffi- 
cult to  detect  without  the  aid  of  microscopic  investigation.  These  may 
increase  in  amount  until  the  rock  passes  over  into  a  gneiss,  and  no 
hard  and  fast  line  can  be  drawn  between  them,  as  previously  stated. 
The  decision  as  to  whether  a  given  rock  should  be  classed  as  a  gneiss 
or  mica-schist  is  often  a  very  difficult  thing  to  make  on  purely  mega- 
Bcopic  grounds;  in  general  if  the  amount  of  mica  is  large,  and  little  or 
no  feldspar  can  be  detected  with  the  lens,  it  is  best  to  classify  it  as  a 


366  ROCKS  AND  ROCK  MINERALS 

mica-schist ;  if  the  amount  of  mica  is  small  and  feldspar  can  be  seen, 
to  define  it  as  a  gneiss. 

On  the  other  hand,  in  proportion  as  the  original  sandstones  were 
more  and  more  purely  composed  of  quartz  grains  there  would  be  less 
and  less  of  mica  made,  and  in  this  way  formal  transitions  into  quartz 
schist  and  quartzite  are  produced  in  their  metamorphic  representa- 
tives. We  thus  see  that  gneiss,  mica-schist,  and  quartzite  form  a 
graded  series  whose  divisional  lines  must  be  purely  arbitrary. 

Again,  as  the  rocks  become  finer  and  finer  in  grain  and  in  texture, 
the  mica-schists  pass  into  micaceous  slates  and  so  on  into  slates,  and 
this  becomes  more  marked  if  the  amount  of  carbonaceous  matter 
increases,  as  it  tends  to  mask  the  mica.  The  divisional  line  thus 
becomes  an  arbitrary  one  in  this  case  also. 

QUARTZITE. 

Quartzite  is  a  firm,  compact  rock,  composed  of  grains 
of  quartz-sand  united  by  a  cement  consisting  of  the  same 
material,  that  is,  of  deposited  quartz.  They  are  in  general 
metamorphosed  sandstones,  and  while  no  hard  and  fast 
line  can  be  drawn  between  the  two  rocks,  since  all  degrees 
of  transition  can  be  found  between  them,  the  quartzites 
are  much  harder  and  firmer  than  the  sandstones;  the 
latter  have  a  more  or  less  sugar-granular  feeling  and 
appearance;  the  individual  grains  are  distinctly  visible  to 
the  eye  or  lens,  while  in  the  quartzites  the  fractured  sur- 
face is  uneven,  splintery  or  conchoidal;  the  luster  vitreous 
or  greasy,  like  that  of  quartz,  and  the  grains  are  imper- 
ceptible or  nearly  so.  This  difference  arises  chiefly  from 
the  fact  that  in  breaking  the  sandstone  the  fracture  takes 
place  in  the  cement,  leaving  the  grains  unaltered  and 
outstanding,  while  in  quartzite  the  grains  are  so  firmly 
cemented,  that  there  is  nearly  a  homogeneous  substance 
formed  and  the  fracture  takes  place  through  cement  and 
grains  alike.  This  difference  will  serve  as  a  practical  dis- 
tinction between  the  two  rocks. 

Minerals  and  General  Properties.  While  some  quartz- 
ites are  very  pure  in  mineral  composition,  others  carry  in 
greater  or  less  abundance  other  minerals,  which  may  be  in 
part  remains  of  original  mineral  grains,  such  as  feldspar 


367 


mixed  with  those  of  quartz,  or  new  ones  which  have 
resulted  from  the  metamorphism  of  the  clay  or  lime 
cement,  which  formerly  filled  the  interstices  between  the 
grains  of  the  sandstone.  Such  are  muscovite,  chlorite, 
cyanite,  epidote,  etc.  Iron  hydroxides  may  be  con- 
verted into  magnetite  or  hematite,  and  carbonaceous 
substance  into  graphite.  These  resultant  minerals  are 
usually  of  microscopic  size,  and  may  give  the  rock  a  dis- 
tinct color  —  green,  blue,  purple,  black,  etc.;  sometimes 
they  are  large  enough  to  be  clearly  seen  with  the  lens. 
The  most  important  of  them  is  muscovite,  which,  as  it 
increases  in  amount,  gives  the  rock  a  more  schistose  char- 
acter, through  which  it  attains  a  capacity  for  cleavage 
along  the  planes  of  the  mica.  Eventually  this  produces  a 
transition  into  mica-schist,  as  previously  explained  under 
that  rock.  The  normal  color  of  quartzite  is  white,  light- 
gray  or  yellowish  into  brown,  but  these  are  often  modified 
by  included  material  acting  as  a  pigment,  as  explained 
above.  The  jointing  of  quartzite  is  usually  platy,  but 
sometimes  very  massive,  and  such  rocks  are  in  some 
places  quarried  and  furnish  good  material  for  structural 
purposes. 

The  chemical  composition  of  a  pure  quartzite  is  nearly 
that  of  silica  alone,  but  as  more  or  less  clay  or  calcareous 
material  was  mixed  with  the  sand,  small  amounts  of 
alumina,  iron,  lime,  and  alkalies  appear.  This  is  illus- 
trated in  the  contrast  of  the  two  analyses  quoted  below. 


SiO2 

A12O3 

Fe203 

FeO 

MgO 

CaO 

NasO 

K2O 

H2O 

Total 

I... 
II.. 

97.71 
74.22 

1.39 
10.61 

1.25 
7.45 

0.85 

0.13 
1.48 

0.18 
0.56 

2.12 

1.08 

1.79 

100.66 
100.16 

I,  Pure  quartzite,  Chickies  Station,  Pa.;  II,  Impure  quartzite, 
Pigeon  Point,  Minn.  Contains  small  quantities  of  feldspar,  mica, 
chlorite  and  magnetite. 


368  ROCKS  AND   ROCK  MINERALS 

Varieties.  The  different  varieties  of  quartzite  are  chiefly  those 
which  are  occasioned  by  the  presence  of  some  included  substance. 
Thus  we  have  epidotic  quartzite,  graphitic  quartzite,  sillimanite- 
quartzite,  and  many  others.  Micaceous  quartzite  is  also  called 
quartz-schist.  In  very  strongly  folded  and  compressed  mountain 
regions  even  pure  quartzite  may  suffer  such  shearing  as  to  break 
and  crush  the  original  grains  and  impose  a  more  or  less  schistose 
structure.  Such  rocks  are  called  stretched  quartzites.  In  some 
places  these  rocks  contain  pebbles,  of  varying  sizes,  which  retain 
their  original  shape  and  are  sometimes  by  pressure  and  shearing 
reduced  to  lenticular,  ovoid,  or  cylindrical  bodies.  These  are  called 
conglomerate-quartzite  and  were  formed  from  gravels,  like  conglom- 
erate-gneisses and  conglomerate-mica-schists. 

Oolitic  quartzite  is  a  variety  consisting  of  rounded  grains,  com- 
posed of  chalcedony,  a  slightly  hydrated  form  of  silica,  deposited 
around  fragments  of  quartz  which  serve  as  nuclei.  It  resembles 
the  roe  of  a  fish,  and  if  the  globules  are  sufficiently  large,  their  con- 
centric structure  can  be  plainly  seen  with  a  lens  on  the  broken  or 
polished  surface  of  the  rock.  Such  quartzites  have  been  found  at 
State  College,  Pa.,  and  in  Sumatra.  Buhrstone  is  a  name  given 
to  a  variety  of  quartzite  which  is  full  of  long,  drawn  out  hollows 
or  pores.  Notwithstanding  the  porosity,  it  is  quite  firm  and  its 
hardness  and  toughness  have  caused  its  use  as  a  millstone.  It  is 
thought  to  have  been  originally  more  or  less  of  a  limestone,  filled 
with  fossils,  which,  by  the  action  of  solutions  containing  silica,  has 
been  converted  into  a  quartzite,  consisting  mostly  of  chalcedony, 
whose  cavities  represent  the  leached  out  fossils.  It  occurs  in 
western  Massachusetts,  Georgia,  South  Carolina  and  in  the  Paris 
Basin  in  France.  It  is  chiefly  of  Tertiary  age. 

Occurrences.  Quartzite  is  a  widely  distributed  rock, 
mostly  among  the  older  metamorphosed  strata.  Thus 
it  is  common  in  eastern  North  America,  in  the  Rocky 
Mountains  Cordillera  and  in  various  localities  in  Europe 
and  other  parts  of  the  world.  The  occurrence  of  some 
special  varieties  has  been  already  mentioned. 

Alteration.  On  account  of  the  insoluble,  unyielding 
nature  of  its  constituent  grains  and  their  cement,  quartzite 
resists  erosion  and  the  atmospheric  agencies  well,  and, 
where  it  is  prominent  in  mountain  regions  and  areas 
undergoing  denudation,  it  forms  prominent  features  of  the 
landscape,  bold  ledges,  cliffs,  castellated  crags,  spires,  etc. 


DESCRIPTION  OF  METAMORPHIC  ROCKS        369 

Eventually  the  rock  breaks  down  into  sandy  soil  of  poor 
quality. 

Distinction  from  Other  Bocks.  Quartzites,  which  are 
very  homogeneous  appearing  rocks,  may  be  confused,  in  the 
outcrop  or  hand  specimen,  with  some  limestones  or  felsites 
of  a  similar  color  and  texture.  From  the  former  they  are 
easily  told  by  a  test  of  the  hardness,  or  by  lack  of  effer- 
vescence with  acid;  from  the  latter,  in  the  field  by  the 
different  mode  of  geological  occurrence,  by  the  cleavage 
of  the  feldspar  if  visible  under  the  lens,  or  by  blowpipe 
test.  It  should  be  remembered  that  the  chief  minerals 
composing  these  three  rocks  are  quartz,  calcite,  and  feld- 
spar respectively,  and  they  should  be  tested  accordingly. 

SLATE   OR   ARGILLITE. 

Slates  are  dense,  homogeneous  rocks,  of  such  fine  tex- 
ture that  the  individual  mineral  particles  composing  them 
cannot  be  distinguished  by  the  eye  or  lens,  and  character- 
ized by  a  remarkable  cleavage,  by  means  of  which  they 
split  readily  into  broad,  thin  sheets,  which,  as  is  well 
known,  may  be  used  for  a  variety  of  purposes. 

The  slates  represent  in  metamorphic  form  the  finest 
material  of  the  land  waste  by  erosion,  which,  among  the 
unmetamorphosed  stratified  rocks,  appears  as  clay, 
shales  of  various  kinds,  etc.,  as  previously  described. 
With  such  material  more  or  less  volcanic  dust  and  debris, 
or  tuffs,  may  be  mingled.  The  cause  of  the  slaty  cleavage 
is  discussed  in  a  following  paragraph.  The  difference 
between  slate  and  shale  has  been  discussed  in  the  de- 
scription of  the  latter  rock. 

Mineral  Composition  and  Other  Properties.  The  mineral 
particles  are  so  fine  in  slate  that  the  composition  from 
the  megascopic  standpoint  is  not  a  matter  of  importance. 
It  may  be  mentioned,  however,  that  since  the  clays,  silts, 
etc.,  from  which  they  are  formed  come  from  a  great  variety 
of  sources,  so  the  microscope  detects  in  them  many  and 
varied  minerals,  the  chief  of  which  are  quartz,  mica, 


370 


ROCKS  AND  ROCK  MINERALS 


chlorite,  carbonaceous  substance,  etc.  The  kaolin  and 
feldspar  particles,  which  one  might  naturally  expect,  are 
rare  and  appear  to  have  been  converted  into  other  min- 
erals. They  not  infrequently  contain  crystals  of  pyrite, 
readily  seen  with  the  eye  or  lens,  which  may  attain  large 
size,  sometimes  as  distinct  crystals,  sometimes  as  concre- 
tions, or  replacing  fossils.  Veins,  lumps,  and  lenses  of 
deposited  quartz  are  also  common  in  them,  those  of  cal- 
cite  more  rare.  The  color  is  chiefly  gray,  to  dark  gray,  to 
black,  according  to  the  amount  of  carbonaceous  substance, 
but  they  are  often  green  from  chlorite,  or  red,  purple, 
yellow,  or  brown,  from  the  oxides  of  iron.  The  surface  of 
the  slaty  cleavage  is  apt  to  have  more  or  less  of  a  silky 
luster,  sometimes  scarcely  perceptible;  the  cross  fracture 
has  a  dull  surface.  While  the  rock  is  firm  and  never 
friable,  it  is  also  rather  soft,  so  that  it  may  be  quite  readily 
cut,  a  feature  of  great  value  for  technical  purposes.  The 
specific  gravity  of  an  average  slate  is  about  2.8.  The 
chemical  composition  is  shown  in  the  following  analyses 
of  typical  examples,  made  in  the  laboratory  of  the  U.  S. 
Geological  Survey. 


SiO2 

A1208 

Fe203 

FeO 

MgO 

CaO 

Na2O 

K2O 

H2O 

C 

XyO 

Total 

I 

59.7 

17.0 

0.5 

4.9 

3.2 

1.3 

1.4 

3.8 

4.1 

0.5 

3.8 

100.2 

II 

67.6 

13.2 

5.4 

1.2 

3.2 

0.1 

0.7 

4.5 

3.3 

_ 

0.7 

99.9 

III 

59.8 

15.0 

1.2 

4.7 

3.4 

2.2 

1.1 

4.5 

3.8 

_ 

4.3 

100.0 

IV 

56.4 

15.3 

1.7 

3.2 

2.8 

4.2 

1.3 

3.5 

4.8 

0.6 

6.5 

100.3 

V 

60.5 

19.7 

— 

7.8 

2.2 

1.1 

2.2 

3.2 

3.3 

— 

— 

100.0 

I,  Black  roofing  slate,  Benson,  Vermont.  XyO  =  TiO2,  P,O5,  CO,, 
FeS2,  etc.;  II,  Red  roofing  slate,  Washington  Co.,  New  York 
State;  III,  Green  roofing  slate,  Pawlet,  Vermont,  CO2,  3.0;  IV, 
Black  roofing  slate,  Slatington,  Pennsylvania,  CO2,  3.7;  FeS2, 1.7;  V, 
Roofing  slate,  Wales. 

The  general  predominance  of  magnesia  over  lime  in  the 
analyses,  as  well  as  the  small  amount  of  the  latter,  shows 


DESCRIPTION  OF  METAMORPHIC  ROCKS         371 

that  the  soluble  lime  silicates  have  been  mostly  dissolved 
out  of  the  silt  in  the  process  of  erosion  and  laying  down 
of  the  sediments.  The  presence  of  carbon  in  the  black 
varieties,  and  of  ferric  iron  in  the  red,  is  to  be  noted. 

Varieties.  Roofing  slates  are  compact,  very  fissile  varieties  which 
split  with  a  smooth,  even  cleavage.  All  the  different  colors  are  used, 
but  the  most  common  is  a  dark  gray.  Some  slates  fade  when  taken 
from  the  quarry,  on  continued  exposure,  through  incipient  altera- 
tion and  the  possibility  of  this  can  only  be  determined  by  practical 
trial.  The  presence  of  pyrite  in  any  notable  quantity  is  very 
prejudical,  as  this  substance  on  exposure  quickly  alters  and 
gives  rise  to  rusty  stains.  The  slates  used  for  blackboards  and 
ciphering  are  the  blackest  and  most  compact  kinds.  Calcareous 
slates  are  those  which  contain  a  good  deal  of  intermingled  calcite, 
or  chalky  material,  which  may  rise  to  30  per  cent  of  the  whole; 
they  represent  slates  which  have  been  formed  from  original 
marls. 

Cleavage  of  Slates  and  its  Origin.  The  cause  of  slaty  cleavage 
has  occasioned  much  speculation  and  has  been  the  subject  of  investi- 
gation, both  experimental  and  mathematical,  as  well  as  geological, 
by  a  number  of  scientists.  From  this  work  it  has  become  clear  that 
it  is  the  result  of  great  pressure  upon  the  material  and  that  the 
planes  of  cleavage  are  at  right  angles  to  the  direction  of  pressure. 
When  the  fine-grained  sediments  are  subjected  to  intense  pressure, 
unevenly  shaped  particles  tend  to  rotate,  so  that  their  longer  axes 
are  perpendicular  to  the  direction  of  pressure ;  they  also  tend  to 
become  flattened  perpendicularly  to  it.  This  tends  to  give  the  rock 
a  grain,  an  arrangement  of  particles,  by  which  it  tends  to  split  more 
readily  along  such  a  direction  than  in  any  other.  Moreover  the  rock 
minerals,  which  naturally  tend  to  be  flattened  or  elongate  in  the 
shape  of  their  particles,  such  as  the  micas,  kaolin,  hornblende, 
chlorite,  etc.,  possess  an  excellent  cleavage  parallel  to  the  elongate  or 
flattened  directions,  and  this  is  a  great  help  in  promoting  the  capacity 
of  the  rock  cleavage.  Slaty  cleavage  is  thus  partly  molecular,  or 
mineral  cleavage,  where  it  passes  through  a  single  mineral  particle, 
and  partly  mechanical  where  it  passes  between  arranged,  unlike 
mineral  particles.  Not  necessarily  all  of  the  minerals  whose  cleav- 
age and  arrangement  induce  the  slaty  cleavage  are  original;  some 
of  them,  micas  for  example,  may  have  been  formed  by  the  meta- 
morphism  accompanying  the  pressure. 

The  planes  of  cleavage  do  not  necessarily  bear  any  definite  rela- 
tion to  those  of  original  bedding.  The  beds  were  laid  down  horizon- 


372  ROCKS  AND  ROCK  MINERALS 

tally  and  the  direction  of  pressure  is  also  usually  horizontal;  the 
cleavage  planes  are  at  right  angles  to  this,  and  may  therefore  cut  the 
bedding  at  right,  or  highly  inclined, 
angles.  But,  as  the  beds  may  be  folded 
before  the  pressures  become  intense, 
the  cleavage  planes  may  pass  through 
the  bedding  at  various  angles,  although 
they  themselves  are  strictly  parallel,  as 
seen  in  the  diagram,  Fig.  74. 

Fig.  74-     Slaty  Cleavage   in          Slates,  in  addition  to  their  cleav- 
Folded  Beds.  '   .    ,  ,    ,    ,  .  .    , 

age,   are  intersected    by  cross   joints 

which  are  frequently  so  numerous  as  to  divide  them  into  small 
blocks  and  prevent  their  technical  use.  They  generally  form  sys- 
tems intersecting  at  definite  angles.  In  the  older  mountain  ranges 
the  slates  are  frequently  crumpled  by  repeated  movement  and  show 
this  upon  their  cleavage  surfaces. 

Occurrence.  Slates  are  common  rocks  in  metamorphic 
regions  and  range  geologically  from  the  Algonkian  up  to 
recent  periods.  In  eastern  North  America  they  are  chiefly 
Paleozoic  and  have  an  extensive  developement  in  Maine, 
in  Vermont,  in  Pennsylvania,  and  in  Georgia.  They  are 
also  extensively  distributed  in  the  Lake  Superior  region 
and  in  the  older  ranges  of  the  Rocky  Mountains  Cordillera. 
They  are  found  in  southern  England,  in  Wales  and  in 
many  other  parts  of  Europe. 

Phyllite.  Closely  connected  with  slate  by  intermediate 
types  are  a  group  of  rocks  to  which  the  name  of  phyllite 
has  been  given.  The  name  means  "  leaf  stone  "  and  is 
used  on  account  of  the  remarkable  cleavage  of  the  rocks, 
by  means  of  which  they  split  into  exceedingly  thin  sheets, 
in  typical  examples.  The  surface  is  sometimes  flat, 
sometimes  curved,  folded,  or  crumpled  by  crustal  move- 
ments. It  differs  from  ordinary  slate  in  containing  a 
larger  amount  of  mica,  or  at  all  events  the  mica  is  in 
larger  flakes,  and  is  more  evident,  giving  the  surface  of 
cleavage  a  shimmering  or  micaceous  appearance,  and  thus 
furnishing  a  transition  form  between  slate  and  mica- 
schist.  The  mica  is  a  fine,  scaly,  silky  variety  of  mus- 
covite  to  which  the  name  of  sericite  has  been  given. 


DESCRIPTION  OF  METAMORPHIC  ROCKS        373 

Quartz  is  the  other  chief  mineral  and  may  sometimes  be 
seen  on  the  cross  fracture.  Rocks,  which  in  this  country 
have  been  called  "  hydromica-schists,"  are  in  large  part 
such  phyllites.  Their  color  is  sometimes  pure  white, 
more  often  tinged  with  reddish,  yellowish,  or  greenish 
tones,  and  sometimes  dark  colored,  or  black,  from  pig- 
ments, like  those  of  slate.  They  are  apt  to  have  a  soft 
talcy  or  greasy  feel,  and  to  be  more  brittle  than  slate,  and 
to  lack  its  toughness  and  firmness.  Sometimes  they  con- 
tain visible  crystals  of  pyrite,  garnet,  and  other  minerals. 

The  origin  of  phyllites,  as  shown  by  the  researches  which  have  been 
made  upon  them,  is  a  varied  one;  in  some  cases  they  represent  sedi- 
mentary material  which  has  been  metamorphosed,  like  the  slates, 
but  has  attained  a  more  complete  degree  of  recrystallization  than 
they  have.  On  the  other  hand  a  considerable  part  of  the  phyllites 
represent  original  felsites  —  igneous  rocks  —  which  have  been 
subjected  to  the  energetic  operation  of  rnetamorphism  through 
dynamic  forces,  to  pressure  and  great  shearing,  aided  probably  by 
liquids  and  heat.  Their  feldspars  have  been  largely,  if  not  entirely, 
converted  into  mica,  and  a  thin  schistose  or  slaty  cleavage  has  been 
imposed  upon  them.  In  some  extreme  cases  the  rock  appears 
as  if  wholly  composed  of  this  silky  mica.  The  chemical  analyses  of 
these  rocks  show  them  to  have  compositions  similar  to  that  of  many 
felsites  or  felsite  tuffs. 

Porphyroid  —  Sheared  Felsites.  In  many  places  where  phyllites 
occur,  they  may  be  traced  into  types  which  are  firmer,  with  less  pro- 
nounced but  yet  distinct  cleavage,  and  which  contain  visible  pheno- 
crysts  of  quartz  and  feldspar,  similar  to  those  in  felsite-porphyries 
(embedded  in  the  phyllitic  ground  mass).  Such  rocks  have  been 
termed  porphyroid.  These  again  may  be  further  traced  into  un- 
doubted felsites  which  still  retain  the  phenocrysts,  flow  structures, 
spherulites,  etc.,  characteristic  of  lavas,  or  the  broken,  angular,  frag- 
mental  features  of  tuffs  and  breccias,  in  spite  of  the  slaty  cleavage, 
which  to  a  greater  or  lesser  degree,  has  been  imposed  upon  them 
by  the  dynamic  movements  and  shearing  to  which  they  have  been 
subjected.  These  again  may  be  followed  into  undoubted,  unsheared 
felsites.  Rocks  with  these  characters,  in  their  varied  types  as 
described  above,  occur  in  various  places  among  the  older  metamor- 
phosed Paleozoic  areas  of  eastern  North  America,  in  Maine,  at  South 
Mountain,  Pa.,  in  Virginia  and  North  Carolina,  in  Wisconsin,  the 
Lake  Superior  region,  etc.  They  have  been  found  of  various  ages 
in  Great  Britain,  Germany,  the  Alps  and  other  places  in  Europe. 


374  ROCKS  AND  ROCK  MINERALS 

In  Sweden  such  ancient  felsites  and  felsite  tuffs,  hardened  and 
more  or  less  metamorphosed,  have  been  termed  hdlleflinta. 

It  is  only  in  comparatively  recent  years  that  such  altered  igneous 
rocks,  with  more  or  less  schistose  appearance  and  cleavage,  have  been 
recognized  and  their  significance  appreciated.  The  older  geologists, 
confused  by  their  cleavage,  regarded  and  mapped  them  as  slates  and 
considered  them  as  of  sedimentary  origin.  They  are  of  interest, 
because,  as  stated  in  the  introduction  to  metamorphic  rocks,  these 
latter  comprise  material  both  of  igneous  and  sedimentary  origin. 
Of  the  feldspathic,  igneous  rocks,  the  coarser-grained  ones,  like 
granite,  as  we  have  seen,  yield  gneisses ;  the  compact  felsites  and  their 
tuffs  under  the  metamorphic  agencies  of  pressure,  shearing,  etc.,  are 
turned  into  phyllites,  porphyroids,  and  compact  slaty  rocks,  according 
to  the  degree  to  which  these  agencies  have  acted.  The  igneous 
ferromagnesian  rocks  we  shall  see  later  among  the  amphibolites 
and  other  schists. 

TALC-SCHIST. 

Talc-schist  is  a  rock  of  pronounced  schistose  cleavage  and 
character,  in  which  talc  is  the  predominant  mineral.  The 
talc  is  present  in  fine  scales  to  coarse  foliated  aggregates. 
Other  minerals  also  occur  in  different  varieties  of  the  rock, 
such  as  quartz  in  grains,  lenses,  and  veins ;  or  magnetite  and 
chromite  in  black  specks  and  grains;  hornblende,  usually 
in  white  or  green  prisms,  or  crystals  of  enstatite;  chlorite 
mingled  with  the  talc,  etc.  The  color  is  usually  light, 
white  to  pale  green,  or  yellowish,  or  gray;  sometimes  dark 
gray  or  greenish.  The  rock  is  soft  and  the  talc  gives  it 
a  greasy  feeling,  and  often  a  pearly  or  tallowy  appearance 
on  the  cleavage  surface.  In  addition  to  its  micaceous 
appearance  and  soft  greasy  feel,  the  talc  is  easily  told  by 
its  infusibility  before  the  blowpipe,  and  its  insolubility  in 
acids.  The  rock  cleavage  is  sometimes  thinly  fissile, 
sometimes  thicker,  and  sometimes  cleavage  is  nearly 
wanting,  the  rock  is  more  nearly  massive,  is  compact,  and 
has  a  lard-like  or  wax-like  aspect,  and  approaches  soap- 
stone  in  character.  Chemically,  these  rocks  consist  mostly 
of  silica  and  magnesia  with  small  amounts  of  water  and 
other  oxides. 


DESCRIPTION  OF  METAMORPHIC  ROCKS 


375 


SiO2 

A12O3 

Fe2O3 

FeO 

MgO 

CaO 

Na2O+K2O 

H2O 

Total 

I.. 

58.7 

9.3 

4.4 

22.8 

0.9 

4.1 

100.2 

II.. 

53.3 

4.4 

5.8 

1.0 

29.9 

1.5 

1.5 

2.6 

100.0 

I,  Talc-schist,  Falun,  Sweden;  II,  Talc-schist,  Zobtau,  Moravia, 
Austria. 

The  composition  is  quite  similar  in  its  general  features 
to  that  of  the  peridotites  among  igneous  rocks,  as  may  be 
seen  by  reference  to  their  analyses. 

The  talc-schists  undoubtedly  represent  material  which  was  some- 
times of  igneous  origin,  peridotite,  pyroxenite,  or  dunite,  and  some- 
times of  sedimentary  origin,  dolomitic,  ferrugineous  marls,  etc.  It 
may  not  be  possible  from  field  work  and  an  inspection  of  specimens 
alone,  unless  aided  by  chemical  analyses  and  microscopical  study, 
to  decide  in  any  given  case  which  origin  the  material  had,  and  some- 
times not  even  then.  The  presence  of  chromium,  either  in  the  form 
of  chromite  or  of  secondary  minerals  derived  from  it,  such  as 
kammererite  or  fuchsite  (a  variety  of  muscovite  green  from  chrom- 
ium), is  indicative  of  igneous  origin,  while  that  of  much  quartz  and 
dolomite  mingled  with  the  talc,  which  produces  the  variety  of  talc- 
schist  called  listwanite,  would  be  on  the  other  hand  more  indicative 
of  a  sedimentary  one. 

Talc-schists  do  not  form  important  formations  like 
gneiss,  mica-schist,  and  slates,  but  are  limited  in  occurrence, 
being  found  as  interbedded  layers  or  inclusions,  chiefly  as 
lenticular  masses,  in  other  metamorphic  rocks,  and  are 
really  not  very  common.  They  show  transitions  in  places 
into  other  rocks,  such  as  chlorite-schist,  crystalline  dolo- 
mite, quartzite,  etc.  Such  transitions,  or  the  lack  of 
them,  may  furnish  useful  hints  in  regard  to  their  origin  in 
particular  cases.  In  eastern  North  America  talc-schists 
occur,  associated  with  other  metamorphic  rocks,  in 
Canada,  in  the  New  England  states,  in  northern  New 
York,  and  south  to  Georgia.  They  are  also  found  in  the 
Rocky  Mountains  region  and  in  the  Pacific  states,  Cali- 


376 


ROCKS  AND   ROCK  MINERALS 


fornia,  Oregon,  etc.  In  Europe  they  occur  in  the  Alps, 
Germany,  and  various  other  places,  in  Sweden,  Finland, 
etc.  They  also  occur  in  Brazil  and  other  parts  of  the  world. 
Their  occurrence,  though  not  generally  of  wide  geologic 
interest,  is  important  because  they  furnish  a  source  of 
supply  for  talc,  which  is  used  for  a  variety  of  purposes. 

CHLORITE-SCHIST. 

These  rocks  are  schists  which  have  the  mineral  chlorite 
as  their  chief  determinant  mineral.  It  occurs  as  fine, 
scaly  aggregates,  sometimes  too  fine  for  the  individual 
scales  to  be  seen  by  the  eye;  more  rarely  in  foliated  to 
coarse  foliated  aggregates.  It  is  sometimes  thinly,  some- 
times thickly,  schistose,  and  in  some  cases  almost  massive; 
and  although  the  rock  is  very  soft  and  may  be  readily  cut, 
it  is  very  tough  in  the  more  massive  varieties.  The  color 
varies  through  different  shades  of  green,  yellow-green,  to 
dark  green.  Different  minerals  are  apt  to  accompany  the 
chlorite,  some  of  which  may  be  in  megascopic  sizes;  of 
these  may  be  mentioned  magnetite,  often  in  fine  crystals; 
hornblende  in  slender  needles  or  prisms;  corundum  and 
cyanite  in  some  cases;  quartz,  which  is  generally  in  veins 
and  lenses;  epidote  in  grains  and  crystals;  in  some  instances 
graphite,  calcite,  dolomite,  etc.  The  chemical  composi- 
tion of  these  rocks  is  very  variable,  so  far  as  is  known, 
for  not  many  have  been  investigated;  it  indicates  that 
they  have  resulted  from  several  different  sources,  as  seen 
in  the  following  analyses. 


SiOa 

A12O3 

Fe2O3 

FeO 

MgO 

CaO 

Na2O 

K2O 

H2O 

XyO 

Total 

49.2 
26.2 

15.1 
23.7 

12.9 
15.7 

14.5 

5.2 
8.3 

10.6 
1.7 

3.6 
0.5 

1.5 
0.6 

1.9 
7.3 

0.8 

100.0 
99.3 

I,   Chlo rite-schist,   east   of  Roton,    Sweden;  II,   Chlorite-schist 
Benguet,  Luzon  Island,  Philippines. 


DESCRIPTION  OF  METAMORPHIC  ROCKS         377 

No.  I  has  a  composition  very  similar  to  that  of  the  group 
of  igneous  rocks  known  as  gabbros,  as  may  be  seen  by 
reference  to  their  analyses,  and  to  which  also  the  dolerites 
and  basalts  belong,  these  being  merely  textural  varieties  of 
magmas  similar  to  gabbros.  No.  II,  on  the  other  hand, 
is  very  different,  and  does  not  correspond  to  any  igneous 
rock;  it  suggests  rather  a  very  ferrugineous  clay. 

The  chlorite-schists  are  of  wide  distribution,  forming 
subordinate  layers  or  masses  in  the  midst  of  gneisses, 
mica-schists  and  other  such  rocks,  characteristic  of  meta- 
morphic  areas.  Thus  they  occur  in  Canada,  New  England, 
New  York,  Pennsylvania,  etc.  They  are  also  common 
in  Europe,  in  the  Alps,  Germany,  Sweden  and  other  places. 

Greenstone.  Transitions  of  chlorite-schist  into  mica- 
schist,  into  slates,  into  schistose  serpentine,  and  into 
hornblende-schist  occur  in  places.  Under  the  descrip- 
tion of  gabbro  and  of  dolerite  it  was  mentioned  that  these 
rocks  by  alteration,  through  processes  of  regional  meta- 
morphism,  passed  into  hornblende-schists  and  into  so- 
called  "  greenstone  "  or  "  greenstone-schist."  In  such 
cases  the  original  ferromagnesian  minerals,  or  the  horn- 
blende produced  from  them,  have  been  largely  changed 
by  alteration  into  chlorite  which  gives  the  rock  its  green 
color.  Such  greenstones  (if  massive),  or  greenstone- 
schists  (if  schistose),  which  thus  represent  altered  dolerites, 
basalts,  and  gabbros,  form  transition  types  to  chlorite- 
schist.  The  alteration  of  hornblende  in  diorites  to 
chlorite  also  produces  greenstones.  It  is  conceivable  that 
a  dolerite  might  pass  directly  by  alteration  into  a  chlorite 
rock,  or  greenstone,  and  thus  be  of  massive  character,  or  it 
might  be  first  changed  into  a  hornblende-schist  and  this 
secondarily  alter  into  a  chlorite-schist.  But  since  horn- 
blende-schists are  produced,  not  only  from  igneous  but 
also  from  sedimentary  beds,  as  described  in  the  account 
of  these  rocks,  the  mere  fact  that  transitions  from  horn- 
blende-schists into  chlorite-schists  occur  does  not  alone 
prove  these  latter  have  been  derived  from  igneous  rocks 


378 


ROCKS  AND  ROCK  MINERALS 


in  any  given  case.  Transitions  from  dolerite  into 
chlorite  rocks,  or  greenstone-schists,  have  been  observed  in 
many  regions;  in  Michigan,  Maryland,  Connecticut,  in  the 
south  of  England,  in  the  Alps,  Germany,  etc.  A  green- 
stone-schist from  the  Menominee  River,  Michigan,  which  is 
known  to  be  an  altered  dolerite,  has  the  following  com- 
position. 


Si02 

A1203 

Fe2O3 

FeO 

MgO 

CaO 

NazO 

K20 

H2O 

CO2 

Total 

44.5 

16.4 

5.1 

5.5 

7.5 

7.9 

2.6 

0.5 

5.0 

5.4 

100.4 

It  is  composed  of  chlorite,  with  some  feldspar,  quartz 
and  calcite.  This  should  be  compared  with  the  analyses 
of  gabbro  which  represent  the  gabbro-dolerite  group. 

The  greenstones  vary  in  color  from  pale  gray-green 
through  yellowish  green  to  dark  green.  The  color  depends 
on  the  proportion  of  chlorite  to  other  minerals.  They  are 
generally  too  compact  for  the  megascopic  determination  of 
the  individual  mineral  particles.  They  are  generally  rather 
soft  rocks.  Sometimes,  when  the  original  dolerite  or 
basalt  was  an  amygdaloid,  this  amygdaloid  structure  is 
retained  and  the  rock  is  filled  with  little  balls  of  calcite 
or  quartz.  In  other  cases,  where  the  rock  has  been  strongly 
sheared,  these  have  disappeared,  but  are  still  represented 
in  the  schist  by  ovoid  spots  of  a  different  color  and  mineral 
composition  from  the  main  mass.  In  some  rare  cases 
they  have  been  replaced  by  ores.  The  amygdaloidal 
structure  is  a  good  proof  of  the  original  igneous  character 
of  the  rock. 

Soapstonc,  Steatite.  As  an  appendix  to  talc  and  chlorite-schists 
may  be  mentioned  soapstone  or  steatite,  a  massive  rock,  usually  of  a 
gray  or  green,  but  sometimes  of  a  dark  color;  the  lighter  colors  often 
with  a  silvery  or  shimmering  fracture  surface.  It  is  very  soft, 
easily  cut  or  worked,  without  cleavage  or  grain,  and  resists  well 
heat  and  the  action  of  acids.  For  these  reasons  it  has  been  exten- 


DESCRIPTION  OF  METAMORPHIC  ROCKS         379 

sively  used  in  prehistoric  times  for  the  manufacture  of  pots  and  other 
vessels,  and  is  employed  at  present  in  table  tops,  sinks,  and  other  inte- 
rior fittings  where  its  qualities  render  it  valuable.  It  is  usually  a 
variable  mixture  of  interwoven  scales  of  talc  and  chlorite  mixed  with 
various  minerals;  in  some  cases  carbonates  are  present.  The  better 
qualities  incline  more  nearly  to  pure  talc.  The  minerals  are  in 
general  too  fine  for  megascopic  determination.  It  occurs  in  con- 
nection with  talc  and  chlorite  rocks,  and  sometimes  with  serpentine, 
in  various  parts  of  the  world,  in  areas  of  metamorphic  rocks. 

AMPHIBOLITE  OR  HORNBLENDE-SCHIST. 

The  amphibolites  are  a  large  group  of  metamorphic 
rocks  whose  distinguishing  characters  are,  that  they  con- 
sist partly  or  largely  of  hornblende,  and  that  they  possess 
a  more  or  less  pronounced  schistose  structure.  There  are 
a  number  of  varieties  in  the  group,  depending  on  the  kind 
of  hornblende  present,  and  on  the  minerals  associated 
with  it,  so  that  it  is  difficult  to  give  a  general  description 
which  will  cover  all  cases.  It  is  best  therefore  to  describe 
the  most  common  kind  first,  and  then  give  a  brief  mention 
of  some  of  the  less  common  varieties. 

Common  hornblende-schists  or  amphibolites  are  rocks 
which  vary  in  color  from  green  to  black;  the  green  is  of 
varying  tones,  clear  light  green,  gray-green,  yellowish 
green  to  dark  green,  greenish  black  to  black;  the  darker 
colors  are  more  common.  The  color  is  given  by  the 
hornblende,  though  in  a  considerable  degree,  in  some 
cases,  it  is  influenced  by  admixed  chlorite.  The  grain  of 
the  rocks  varies  from  coarse  to  fine,  the  latter  being  more 
common.  When  coarse,  the  hornblende,  which  is  almost 
always  present  in  slender  prisms  or  blades  and  rarely  in 
grains,  is  easily  recognized  by  the  eye  from  its  form  and 
bright,  good  cleavage.  In  such  cases  the  prisms  may  be 
an  inch  or  more  in  length  and  have  the  thickness  of  a 
slender  match  stick;  from  this,  in  the  finer-grained 
types,  they  sink  to  tiny  needle-  or  hair-like  prisms  which 
can  only  be  seen  by  careful  observation  with  a  good 
lens.  The  prisms  are  usually  arranged  in  the  direction  of 


380  ROCKS  AND  ROCK  MINERALS 

schistosity  and  thus  approach  parallel  positions;  it  is  this 
which  chiefly  gives  the  rock  its  cleavage.  It  also  gives 
the  rock,  especially  in  the  finer-grained  types  with  needle- 
like  prisms,  a  shimmering  or  silky  luster  on  cleavage  sur- 
faces, which  is  rather  characteristic.  In  some  cases  the 
grain  is  so  extremely  fine  that  not  even  with  the  lens 
can  the  individual  minerals  be  seen;  such  rocks  may 
appear  very  much  like  slates,  and  are  indeed  difficult  to 
distinguish  from  them;  they  are  however,  not  very  com- 
mon types. 

The  amphibolites  are  rather  hard  rocks,  not  easily 
scratched  by  the  knife.  In  the  more  schistose  types 
they  are  brittle,  but  as  they  become  more  massive  in 
character  they  are  very  tough  and  difficult  to  break. 
They  are  heavy,  the  specific  gravity  ranging  from  3.0-3.4. 

In  addition  to  the  hornblende  other  minerals  are  present  in 
varying  kinds  and  quantities;  prominent  ones  are  quartz,  feldspar, 
and  mica.  The  quartz  and  feldspar  in  grains  are  best  observed  with 
the  lens  on  the  cross  fracture;  often  they  are  too  fine  and  too  much 
masked  by  the  hornblende  to  be  seen;  the  quartz  also  at  times 
forms  little  lenses  or  masses,  or  fills  fractures  in  the  shape  of  veins,  as 
in  other  metamorphic  rocks,  and  has  then  been  secondarily  deposited 
from  solutions.  The  mica  can  be  generally  seen  on  the  surface  of 
chief  fracture ;  both  biotite  and  muscovite  occur  and  may  increase 
to  such  an  extent  as  to  produce  formal  transitions  to  mica-schist. 

Other  minerals  which  may  be  detected  megascopically  are  iron 
ore,  pyrite,  garnet  in  small  dark  red  crystals,  chlorite,  calcite;the 
latter  sometimes  in  veins,  etc.,  like  quartz.  Pyroxene,  epidote,  and 
other  minerals  occur,  but  partly  on  account  of  the  fineness  of  grain, 
and  partly  on  account  of  their  resemblance  to  hornblende,  it  is 
usually  impossible  to  detect  and  identify  them  without  microscopic 
study. 

The  chemical  composition  of  amphibolites  has  not  yet 
been  as  thoroughly  investigated  as  it  should  be,  but 
what  has  been  done  shows,  in  agreement  with  facts 
to  be  presently  mentioned,  that  the  origin  of  these 
rocks  is  various.  The  following  analyses  will  do  for 
examples. 


DESCRIPTION  OF  METAMORPHIC  ROCKS         381 


I.. 

TT 

SiOj 

49.9 
50.4 

A12O3 

Fe2O3 

FeO 

MgO 

CaO 

NajO 

K20 

H20 

XyO 

Total 

15.5 
13.3 

3.0 
6.3 

8.0 

9  3 

7.8 
5  6 

8.9 
7.9 

3.3 
2.1 

0.7 
1  1 

1.5 
1  7 

1.7 

?,  0 

100.3 
99.7 

III. 
IV  . 
V 

55.0 
55.6 
45.6 

2.9 
16.3 
14.2 

0.8 
1.2 
1.2 

6.3 

7.2 
9  8 

21.0 
5.6 
6  8 

11.5 
9.2 
2.3 

0.3 
0.9 
1.6 

0.2 
0.2 
1  ?, 

1.0 
3.1 
5  1 

1.5 
1.0 
1?  fi 

100.5 
100.3 
100.4 

VI. 

52.4 

13.6 

2.7 

9.8 

5.5 

10.0 

2.3 

0.4 

1.7 

1.6 

100.0 

I,  Thin  schistose  amphibolite,  Whitman's  Ferry,  Sunderland,  Mass.; 
II,  Amphibolite,  Crystal  Falls  district,  Michigan;  III,  Grass-green 
Amphibolite,  Chiavenna.  XyO  =  Cr2O3;  IV,  Amphibolite,  Goshen, 
Massachusetts;  V,  Amphibolite,  pyritiferous,  Conrad  Tunnel,  Ophir 
district,  California.  XyO  =  Pyrite  7.9,  CO2  3.0,  TiO2l.l,  plus  traces; 
VI,  Olivine-basalt,  main  lava  flow,  Pine  Hill,  South  Britain,  Con- 
necticut. 

Of  these  analyses  Nos.  I  and  II  have  compositions  very  much 
like  that  of  the  gabbro-basalt  magmas,  as  may  be  seen  by  comparison 
with  No.  VI ;  No.  Ill  has  the  general  composition  of  the  peridotite- 
pyroxenite  group  of  igneous  rocks  and  may  be  compared  with 
Analysis  No.  Ill  of  that  group.  The  presence  of  Cr2O3  in  III  is  also 
significant  of  an  igneous  origin.  On  the  other  hand  No.  IV  is 
thought  on  geological  grounds  to  be  derived  from  an  impure  lime- 
stone, probably  full  of  clay,  and  this  supposition  is  rendered  probable 
by  the  fact  that  the  high  alumina  is  accompanied  by  an  almost 
entire  lack  of  alkalies,  a  feature  not  seen  in  igneous  rocks.  In  No.  V, 
while  alkalies  are  present  with  the  alumina,  high  magnesia  and 
ferrous  iron  are  not  accompanied  by  high  lime  and  these  make  a 
combination  not  seen  in  igneous  rocks.  Compare  again  No.  VI. 
This  rock,  No.  V,  is  probably  derived  from  an  impure  ferrugineous 
arkose  or  silt. 

Origin  of  Aonphibolites.  As  just  shown,  the  composition 
of  amphibolites  sometimes  corresponds  with  that  of 
igneous  rocks,  and  sometimes  does  not,  and  this  agrees 
with  the  results  of  geological  investigation  in  the  field. 
For  in  some  places  we  find  them  under  conditions  which 
strongly  suggest  their  derivation  from  igneous  rocks,  and 
in  other  places  such  evidence  is  either  wanting,  or  the 
contrary  is  indicated.  The  use  of  the  microscope  on  thin 


382  ROCKS  AND  ROCK  MINERALS 

sections,  by  which  the  inward  textures  and  associated 
minerals  may  be  seen,  also  leads  to  the  same  conclusions. 

Under  the  description  of  gabbro  and  dolerite  it  was 
mentioned  under  alteration,  how  these  rocks  by  pressure 
and  shearing  became  converted  into  hornblende-schist  or 
amphibolite.  Gradual  transitions,  without  geological 
break,  from  one  kind  into  th.e  other,  are  found.  Thus  as 
the  feldspathic  igneous  rocks  give  rise  to  gneisses,  phyllites, 
etc.,  so  the  ferromagnesian,  especially  the  pyroxenic, 
igneous  rocks  give  rise  especially  to  hornblende-schists, 
and  also  to  talc-schists,  chlorite-schists,  and  to  serpentine. 

Sedimentary  beds  of  impure  mixed  character,  such  as 
limestones  containing  sand,  clay,  and  more  or  less  of  the 
hydroxides  of  iron,  limonite,  etc.,  or  marls  of  a  somewhat 
similar  nature,  if  subjected  to  metamorphism  might,  under 
suitable  conditions,  be  converted  largely  into  hornblende, 
mixed  with  other  minerals.  In  this  case  the  volatile  con- 
stituents—  the  water,  carbon-dioxide,  etc., — are  mostly 
driven  out;  the  bases,  lime,  iron,  magnesia  and  alumina, 
combine  with  the  acid  silica,  to  form  silicates,  of  which 
hornblende  is  the  chief  and  determinant  one  of  the  result- 
ing rock.  Thus  hornblende-schists  result  from  the  meta- 
morphism of  sedimentary  strata,  and  may  be  one  form  of 
the  alteration  of  limestone,  as  described  later  under 
marble. 

Varieties  of  Amphibolite.  In  the  midst  of  gneisses  and  mica- 
schists  amphibolite  sometimes  assumes  a  very  massive  character. 
The  prisms  and  grains  of  hornblende,  instead  of  being  arranged  in 
parallel  position,  and  thus  producing  a  schistose  cleavage,  are  inter- 
woven without  arrangement  and  cleavage  is  wanting.  Especially 
in  such  massive  types  is  the  hornblende  liable  to  be  accompanied  by 
feldspar.  If  th?  feldspar  should  increase  and  dominate,  transitions 
to  hornblende  gneiss  would  be  produced.  There  is  a  tendency  on 
the  part  of  some  to  restrict  the  term  amphibolite  to  such  massive 
varieties  and  to  use  hornblende-schist  for  those  with  distinct  cleavage, 
but  this  distinction  has  not  yet  come  into  general  use. 

Eclogite  is  a  variety  of  hornblende-schist  of  a  rather  light  green 
color  sprinkled  full  of  red  garnets.  In  the  typical  examples  of  this 
rock  the  hornblende  is  accompanied,  or  more  or  less  replaced,  by 


DESCRIPTION  OF  METAMORPHIC  ROCKS         383 

a  green  pyroxene.  Other  minerals  also  occur  in  subordinate  amount. 
It  has  been  found  in  various  places  in 'Europe,  and  has  recently 
besn  described  as  occurring  in  a  Californian  locality.  A  closely 
related  hornblende-schist  full  of  garnets  is  found  also  in  Hanover, 
New  Hampshire. 

Glaucophane-schist  is  a  variety  in  which  ordinary  hornblende 
is  replaced  by  the  soda-bearing  species  glaucophane,  and  for  this 
reason  the  rock  is  colored  blue  instead  of  green.  Various  other 
minerals  may  be  present,  depending  on  the  occurrence,  such  as  quartz, 
epidote,  pyroxene,  chlorite,  garnet,  etc.  Sometimes  they  are  coarse 
grained  and  these  other  minerals  may  be  seen,  sometimes  dense  and 
appear  as  slaty  blue  or  blue-gray  rocks.  Studies  which  have  been 
made  of  them  show  that  sometimes  they  have  been  produced  by  the 
metamorphism  of  sedimentary,  sometimes  of  igneous,  material. 
While  comparatively  rare  they  have  been  found  widely  distributed, 
in  California,  Brittany  in  France,  the  Alps,  Island  of  Syra,  Greece, 
Japan,  Australia,  etc. 

Greenstone-schist  in  its  relation  to  amphibolites  has  been  already 
mentioned  under  chlorite-schist.  As  the  use  of  the  term  "  green- 
stone "  has  been  vague,  applying  rather  to  color  than  to  a  deter- 
mined mineral  composition,  many  rocks  which  are  hornblende- 
schists,  rather  than  chlorite-schists,  have  been  included  under  it. 

Occurrence  of  Amphibolites.  These  form  layers  and 
masses  in  other  metamorphic  rocks,  especially  in  gneisses 
and  mica-schists,  rather  than  extensive  independent  for- 
mations. -They  often  occur  in  gneisses  in  long  bands  or 
veins  in  such  a  manner  as  to  suggest  that  they  are  meta- 
morphosed dikes  of  doleritic  rock.  In  size  the  masses  may 
vary  within  the  widest  bounds,  from  one  foot  to  thousands 
in  diameter.  In  some  places,  what  they  lack  in  size,  they 
make  up  in  frequency  of  occurrence.  The  manner  in 
which  they  are  interlaminated  in  places  with  other  meta- 
morphic rocks  suggests  that  they  may  have  been  some- 
times formed  from  intrusive  sheets  of  igneous  rock,  and 
sometimes  from  interbedded.  sediments,  but  in  general 
this  can  only  be  rendered  certain  by  further  chemical  and 
microscopical  investigation.  Their  occurrence  as  mantles 
around  igneous  masses  has  been  already  mentioned. 

The  amphibolites  are  extremely  common  rocks,  in  all 
metamorphic  regions.  Thus  they  are  found  commonly 


384  ROCKS  AND  ROCK  MINERALS 

distributed  in  New  England  and  New  York  State,  and 
southward  to  Georgia;  in  Canada,  the  Lake  Superior  region, 
the  Sierras,  in  England,  Scotland,  the  Alps,  etc. 

Alteration.  It  has  been  already  pointed  out  under 
chlorite-schists  that  the  hornblende  of  these  rocks  may  be 
changed  into  chlorite.  In  another  form  of  alteration  it 
may  be  turned  into  serpentine  with  other  minerals,  and 
thus  give  rise  to  serpentine  rocks,  whose  character  is 
described  later.  These  changes  take  place  in  the  upper 
belt  of  metamorphism,  that  of  hydration  and  cementa- 
tion, and  are  secondary  to  the  processes  which  have  pro- 
duced the  amphibolite  from  something  else.  They 
might  thus  be  spoken  of  as  tertiary  changes. 

By  the  ordinary  process  of  weathering  on  the  surface, 
these  rocks  change  to  masses  of  limonite,  clay,  calcite,  etc., 
which  form  ferrugineous  soils. 

MARBLE  AND  CARBONATE-SILICATE  ROCKS. 

Marble  is  the  metamorphic  condition  of  sedimentary 
rocks  which  are  composed  of  carbonate  of  lime,  CaCOs, 
and  which  in  their  ordinary  stratified  form  are  known  as 
limestone,  chalk,  etc.  It  is  distinguished  from  them  by 
its  crystallization,  coarser  grain,  compactness  and  purer 
colors.  But  just  as  we  have  ordinary  limestones  which 
contain  only  carbonate  of  lime,  and  dolomitic  limestones 
which  contain  magnesian  carbonate,  MgC03,  in  variable 
quantity  associated  with  the  lime  carbonate,  so  we  have 
lime  marbles  and  dolomite  marbles.  As  this  distinction 
is  a  purely  chemical  one  which  is  rarely  made,  and  indeed 
rarely  can  be  made  in  ordinary  and  commercial  usage,  the 
rock  is,  therefore,  called  marble,  without  regard  to  whether 
it  contains  magnesia  or  not.  But  geologically,  especially 
from  the  petrographical  standpoint,  there  is  an  important 
difference  between  the  two  rocks  in  respect  to  the  asso- 
ciated minerals  they  are  apt  to  contain  when  impurities 
were  present  in  them  originally,  and  therefore  they  are 


DESCRIPTION  OF  METAMORPHIC  ROCKS         385 

treated  separately  in  this  work  for  reasons  which  will 
presently  appear. 

General  Properties.  Marble  is  a  crystalline  granular 
rock  composed  of  grains  of  calcite;  sometimes  these  are 
cemented  by  a  fine  deposit  of  calcite  between  them. 
The  grain  varies  from  very  coarse  to  such  fine  compact 
material  that  individual  grains  cannot  be  distinguished;  in 
the  coarsest  varieties,  the  cleavage  surfaces  of  individuals 
may  attain  a  breadth  of  half  an  inch  or  more,  but  this 
is  unusual.  The  fracture  surface  of  the  finest-grained 
kinds  has  a  soft,  shimmering  luster,  while  the  appearance 
of  coarser  kinds  is  like  that  of  loaf  sugar.  The  normal 
color  is  white,  like  that  of  the  best  statuary  marble,  but 
the  rock  is  usually  more  or  less  colored  by  various  sub- 
stances which  act  as  a  pigment,  the  principal  ones  being 
carbonaceous  matter  and  the  oxides  of  iron.  It  thus 
becomes  gray,  yellow,  red  or  black,  and  while  the  color  is 
sometimes  uniform,  it  is  more  generally  spotted,  blotched, 
clouded  or  veined,  producing  that  effect  which  is  known 
as  "  marbled."  The  hardness  is  that  of  calcite,  3;  the  rock 
is  thus  readily  scratched  or  cut  by  the  knife,  a  ready  means 
of  distinction  from  quartzite  or  sandstone,  which  may 
resemble  it.  It  is  readily  soluble  in  weak  acids.  Unless 
the  grain  is  too  fine,  the  good  rhombohedral  cleavage  of  the 
calcite  grains  can  be  easily  seen  with  a  lens.  The  chemi- 
cal composition  of  a  perfectly  pure  marble  would  be  that 
of  calcite,  CaO  =  56,  CO2  =  44  per  cent,  but  there  are 
usually  small  quantities  of  magnesia,  alumina,  iron  and 
silica  present,  coming  from  traces  of  sand,  clay,  dolomite, 
etc.,  mixed  with  it;  these  may  increase  until  the  impure 
marbles  are  produced,  which  are  described  in  a  later 
paragraph. 

Unlike  most  metamorphic  rocks,  marble,  if  pure,  is  very 
massive  and  shows  no  sign  of  schistose  cleavage,  even 
where  its  association  with  schists  is  such  as  to  indicate 
that  it  must  have  been  subjected  to  enormous  pressure 
and  shearing  stresses.  If  impurities  in  the  form  of  other 


386  ROCKS  AND  ROCK  MINERALS 

minerals  are  present  it  may  then  assume  cleavage,  caused 
by  their  presence.  The  reason  for  this  want  of  cleavage 
has  caused  much  speculation;  it  is  probably  due  to  several 
causes,  to  the  purity  of  the  rock,  to  a  rolling  of  the  grains 
among  themselves,  but  chiefly  to  a  curious  property  which 
calcite  possesses  of  permitting  movement  among  its  mole- 
cules, whereby  new  crystal  forms  are  produced  without 
destruction  of  its  substance;  this  results  in  a  complicated 
microscopic  twinning,  somewhat  similar  to  that  explained 
under  feldspar.  As  a  result  of  this  the  stresses  are 
absorbed  molecularly,  instead  of  producing  changes  in  the 
outward  structure,  as  in  most  rocks. 

Varieties  of  Marble.  The  varieties  of  marble  from  the  technical 
point  of  view  are  chiefly  those  which  are  based  on  color.  Statuary 
marble  is  the  purest  and  whitest  kind.  Architectural  marbles  are  those 
of  the  most  uniform  tones  of  color,  while  ornamental  marbles  are 
those  distinguished  by  striking  effects  of  varied  colors,  as  mentioned 
above.  In  the  trade,  the  term  marble  is  used  for  any  lime  carbonate 
or  dolomite  rock  which  can  be  procured  in  large,  firm  blocks,  and  is 
susceptible  of  a  high  polish ;  under  this  definition  many  limestones  are 
included.  Shell  marble  is  thus  a  hard,  firm  limestone  in  which  a 
certain  pattern  is  given  by  the  presence  of  certain  fossils,  shells  of 
brachiopods  and  remains  of  crinoid  stems  being  the  most  common. 
The  different  yellow,  red,  and  black  marbles,  most  of  them  veined 
and  clouded,  of  Italy,  Greece,  and  the  East  have  long  been  distin- 
guished by  a  host  of  names. 

Those  varieties  which  depend  on  the  presence  of  some  mineral, 
additional  to  the  calcite,  are  treated  in  the  following  section  on  car- 
bonate-silicate rocks. 

Occurrence  of  Marble.  The  great  deposits  of  marble, 
from  which  the  material  used  for  structural  purposes  is 
taken,  are  the  result  of  regional  metamorphism  and  it 
is  thus  found  in  regions  of  metamorphic  rocks  associated 
with  gneisses,  schists,  etc.,  in  the  form  of  interbedded 
masses,  layers,  or  lenses.  These  vary  in  size  within  wide 
bounds,  from  a  few  feet  to  many  miles  in  length.  It 
forms  immense  interbedded  layers,  or  masses,  in  the 
Laurentian  rocks  of  Canada;  it  occurs  in  quantities  in 


DESCRIPTION  OF  METAMORPHIC  ROCKS         387 

Vermont,  Massachusetts,  Georgia  and  Tennessee,  where 
it  is  extensively  exploited,  in  Colorado  and  other  places 
in  the  west.  The  marbles  of  Greece  and  Italy  have  at- 
tained celebrity  from  their  use  by  the  ancient  Greeks  and 
Romans  in  statuary  and  buildings.  It  is  found  in  the 
Alps,  Germany,  and  Scandinavia  in  Europe,  and  in 
various  other  places  in  the  world.  Marble  is  also  produced 
from  limestone  (and  chalk)  by  the  contact  action  of  in- 
truded igneous  rock.  Although  some  very  coarse-grained 
material  may  be  formed  in  this  way,  it  is  usually  quite 
limited  in  amount. 

Lime  Carbonate-Silicate  Rocks.  As  described  under 
the  general  properties  of  sedimentary  rocks,  all  transitions 
occur  between  limestones  and  sandstones,  between  lime- 
stones and  shales,  and  between  the  three  combined.  This 
means  merely,  that  the  original  lime  deposits  may  have 
had  sand,  clays,  silt,  and  ferrugineous  material  in  variable 
amounts,  mixed  with  them.  Chemically,  it  means  that 
the  carbonate  of  lime  has  silica,  the  oxides  of  aluminum 
and  iron,  and  usually  small  amounts  of  other  things,  such 
as  magnesia,  potash,  and  soda  mixed  with  it.  Under  the 
conditions  of  metamorphism  the  carbon  dioxide,  C02, 
is  driven  out,  to  be  replaced  by  an  equivalent  amount  of 
silica,  SiO2,  and  thus  silicates  of  lime,  of  lime  and  alumina, 
of  lime  and  iron,  or  mixtures  of  these,  or  combinations 
containing  other  elements  as  well,  are  formed.  Also 
volatile  substances,  liquids  and  gases,  such  as  water 
vapor  furnishing  hydroxyl,  fluorine,  boron,  etc.,  emana- 
tions from  magmas  resting  below  or  being  intruded  simul- 
taneously with  the  crustal  movements  which  give  rise  to 
the  metamorphosing  conditions,  may  enter  the  rock  mass 
and  thus,  in  adding  new  substances,  produce  additional 
mineral  combinations.  The  amount  of  silica  present  may 
be  sufficient  to  completely  replace  the  carbon  dioxide  and 
the  resulting  rock  is  then  composed  entirely  of  silicates, 
or  it  may  not  be  sufficient  to  accomplish  this,  and  the 
mass  then  consists  of  a  mixture  of  lime  carbonate,  calcite, 


388  ROCKS  AND  ROCK  MINERALS 

mixed  with  silicates.  Thus  all  transitions  may  be  found 
from  pure  marble,  through  varieties  containing  bunches, 
masses,  and  individual  crystals  of  some  mineral,  or  miner- 
als, into  rocks  completely  made  up  of  sometimes  one  sili- 
cate, but  usually  of  a  mixture  of  them.  The  whole  affair 
is  quite  analogous  to  what  has  already  been  described  as 
the  effect  of  contact  metamorphism  of  igneous  rocks  on 
impure  limestones  in  a  previous  part  of  this  book,  and  the 
chemical  reactions  which  take  place  are  the  same  as  those 
there  mentioned.  The  resulting  rocks  are  also  quite  simi- 
lar, with,  however,  one  difference.  In  contact  metamor- 
phism the  chief  agency  is  heat,  while  pressure  and 
shearing  are  either  wanting,  or  are  relatively  of  slight 
importance,  but  in  regional  metamorphism  these  are 
factors  of  great  intensity.  Thus  the  rocks  of  contact 
metamorphism  are  massive  and  with  little  or  no  schistose 
cleavage,  while  those  produced  by  regional  metamorphism 
may  strikingly  exhibit  it;  that  cleavage  is  not  always 
present  is  due  to  the  reason  given  above  under  the  des- 
cription of  marble. 

Important  minerals  which  thus  occur  in  limestone  are 
pyroxenes  (especially  wollastonite,  CaSiOs,  and  diopside, 
CaMgSi206) ;  garnets  (especially  grossularite,  Ca3Al2(Si04)3)  ; 
hornblendes  (especially  tremolite,  CaMg3Si4012) ;  feldspar 
(especially  anorthite,  CaAl2Si20g);  vesuvianite;  epidote; 
fluorite,  etc.  A  whole  host  of  minerals  occurs,  but  many 
of  them,  such  as  graphite,  magnetite,  spinel,  titanite, 
tourmaline,  apatite,  phlogopite,  etc.,  come  chiefly  from 
the  impurities  in  the  original  rock,  which  have  been 
recrystallized. 

It  is  clear  from  this,  that,  depending  on  mineral  com- 
bination, a  great  variety  of  these  lime  carbonate-silicate 
rocks  exist,  but  only  a  few  of  the  most  important  types 
can  be  mentioned. 

Wollastonite-rock.  Marble  not  infrequently  contains  crystals  of  the 
pyroxene-like  mineral,  wollastonite,  CaSiO3,  and  this  may  increase 
until  the  rock  is  practically  composed  of  it.  It  is  apt  to  be  accom- 


DESCRIPTION   OF  METAMORPHIC  ROCKS         389 

panied  by  diopside,  hornblende,  etc.  The  rock  is  white,  generally 
massive,  and  resembles  marble,  from  which  it  is  easily  distinguished 
by  its  superior  hardness.  It  occurs  in  California,  the  Black  Forest, 
Brittany,  etc. 

Garnet-rock.  This  is  a  granular  aggregate  of  grains  of  garnet, 
generally  accompanied  by  various  other  minerals  in  smaller,  variable 
amounts.  If  some  calcite  is  yet  present  the  garnets  may  show  more 
or  less  crystal  form ;  sometimes  the  calcite  has  been  leached  out  and 
the  rock  is  porous.  Apt  to  be  yellowish,  to  reddish  brown,  in  color. 
Considerable  magnetite  is  often  present.  New  England,  northern 
New  York,  Montana,  Germany,  Alps,  etc. 

Epidote-rock,  or  Epidosite.  Composed  chiefly  of  epidote  with 
other  minerals,  quartz,  garnet,  etc.  Sometimes  massive  granular, 
sometimes  schistose.  Greenish  in  color,  especially  of  a  yellow- 
green.  Often  very  tough  under  the  hammer.  New  England,  Brazil, 
Germany,  etc.  Sometimes  the  ferromagnesian  igneous  rocks,  basalt 
and  dolerite,  under  proper  metamorphic  conditions,  are  converted 
into  a  rock  consisting  chiefly  of  epidote,  instead  of  hornblende  or 
chlorite  as  previously  described,  and  of  a  yellowish  green  color. 
They  may  resemble  the  above,  but  can  usually  be  distinguished  by 
their  mode  of  occurrence,  geologic  relations,  greater  uniformity,  and 
often  by  the  remains  of  special  structures,  such  as  the  amygdaloidal. 
Instances  occur  in  Pennsylvania,  Virginia,  etc. 

Pyroxene-rock.  In  this  case  the  rock  consists  chiefly  of  pyroxene, 
of  which  the  variety  diopside  is  prominent.  Other  minerals,  quartz 
or  calcite,  etc.,  may  occur.  White,  greenish,  to  dark  green  in  color, 
massive  or  schistose.  Is  found  in  Massachusetts,  northern  New  York, 
Germany,  Bohemia,  Sweden,  etc.  Under  the  head  of  metamorphic 
pyroxene  rocks  there  may  be  mentioned  in  this  connection  jade, 
which,  although  extremely  rare,  is  of  great  interest  from  its  eth- 
nological and  artistic  importance.  Jade  is  a  fine-grained,  and 
usually  compact,  aggregate  of  grains  and  fibers  of  the  soda-pyroxene, 
jadeite,  NaAlSi2O6.  It  is  sometimes  snow-white,  resembling  marble, 
but  usually  greenish  (or  with  a  violet  shade)  to  dark  green.  The 
greenish  colors  are  also  clouded,  veined,  or  specked  through  the 
white.  When  polished  it  has  a  soft,  somewhat  greasy  luster.  The 
extraordinary  toughness  of  the  rock  is  one  of  its  most  marked 
characters  and  on  this  account  it  was  greatly  prized  in  the  early 
history  of  mankind,  before  the  discovery  of  metals,  for  the  manu- 
facture of  weapons  and  implements,  as  shown  by  its  distribution  in 
these  forms,  and  in  unworked  pieces  over  the  world.  It  has  long 
been  greatly  valued  by  the  Chinese,  who  have  devoted  the  most 
laborious  work  to  fashioning  it  into  objects  for  personal  adornment 
and  use.  These  objects,  such  as  vases,  bowls,  etc.,  are  often  carved 
with  wonderful  skill  and  taste  and  are  greatly  prized  for  their 


390  ROCKS  AND  ROCK  MINERALS 

artistic  value.  The  rock  is  only  known  in  place  in  upper  Burma 
and  in  the  Kuen-lun  Mountains  of  Turkestan.  Its  origin  is  uncertain, 
but  its  chemical  composition  suggests  that  it  may  be  a  metamor- 
phosed igneous  rock  of  high  soda  content,  such  as  nephelite-syenite. 
A  green  hornblende  rock  called  nephrite,  from  Siberia  and  New 
Zealand,  has  similar  properties  and  uses  and  is  frequently  mistaken 
for  jade. 

Cipolin  is  a  marble  full  of -mica,  which  may  show  transitions  to 
calcareous  mica-schist.  Usually  other  minerals,  sometimes  in  con- 
siderable variety,  are  also  present. 

Dolomite  Marble,  Magnesia-Silicate  Rocks.  As  men- 
tioned under  dolomite  limestones,  the  rock  name  does  not 
necessarily  mean  that  the  substance  composing  it  is  pure 
dolomite,  in  the  mineralogical  sense.  There  is  generally  an 
excess  of  lime  carbonate  present,  so  that  the  composition 
is  a  mixture  of  dolomite,  MgCa(C03)2,  and  calcite,  CaCO3. 
Just  as  marble  is  related  to  ordinary  limestone,  so  is 
dolomite  marble  to  ordinary  dolomite.  In  a  practical 
way  no  distinction  can  be  drawn  between  the  two  vari- 
eties of  marble,  except  chemically.  See  dolomite  under 
the  rock  minerals.  Like  ordinary  marble,  dolomite  is 
one  end  of  a  series  of  metamorphic  rocks,  which, 
beginning  with  a  pure  carbonate,  becomes  a  mixture  of 
carbonates  and  silicates,  and  ends  in  pure  silicate  rocks. 
The  causes  and  processes  are  identical  with  those 
described  under  marble,  only  in  this  case  the  presence  of 
magnesia  causes  the  formation  of  silicate  minerals,  in 
which  this  element  is  either  the  only  metal,  or  a  vsry 
important  one.  Thus  in  distinction  to  the  lime  carbonate- 
silicate  series,  this  may  be  called  the  magnesian  carbonate- 
silicate  series.  The  magnesian  silicates  thus  produced 
in  the  zone  of  constructive  metamorphism  may  be  anhy- 
drous, or  nearly  so;  on  the  rocks  rising,  by  erosion  or  other- 
wise, into  the  zone  of  hydration,  they  may  be  secondarily 
converted  in  serpentine,  H4Mg3Si209,  or  sometimes  into 
talc,  H2Mg3(Si03)4.  Thus  these  rocks  are  in  many  cases 
closely  connected  with  the  talc-schists  previously  described, 
while  their  relation  to  serpentine  is  mentioned  under  that 


DESCRIPTION    OF    METAMORPHIC    ROCKS        391 

rock.  The  more  important  magnesia  silicates  which  take 
part  in  the  series  are  olivine,  enstatite,  chrondrodite,  diop- 
side,  tremolite,  phlogopite,  etc.,  and  secondarily  serpen- 
tine and  talc,  as  stated  above.  Of  the  varied  rocks  formed 
by  these  mixtures,  only  a  few  of  the  most  important  can 
be  mentioned. 

Crystalline  dolomites,  or  dolomitic  marbles  filled  with  variable 
mixtures  of  minerals,  chrondrodite,  phlogopite,  pyroxenes,  etc.,  with 
others,  such  as  magnetite,  spinel,  apatite,  graphite,  etc.,  coming  from 
original  impurities,  are  found  rather  commonly  in  the  metamorphic 
areas  in  the  eastern  United  States  and  Canada,  but  have  received  no 
distinctive  names,  as  rocks.  They  appear  to  have  been  formed  some- 
times by  contact,  sometimes  by  regional  metamorphism,  often  by  a 
combination  of  both. 

Ophicaldte  is  a  mixture  of  white  calcite  and  green  serpentine,  the 
latter  often  in  veins,  spots,  or  clouded  through  the  rock.  A  part  of 
the  "  verde  antique  "  marble  of  the  ancients,  used  for  ornamental 
purposes,  appears  to  have  been  a  variety  of  ophicalcite.  It  occurs 
in  Canada,  northern  New  York,  and  various  places  in  Europe. 

Soapstone  and  talcschist.  Part  of  the  rocks  included  under  these 
names  belong  in  this  series:  they  have  been  already  described 
under  a  previous  section.  Listwanite,  which  occurs  in  the  Ural 
Mountains,  and  in  Spain,  is  a  mixture  of  magnesia  carbonates  (mag- 
nesite,  MgCO3,  and  dolomite),  with  talc,  and  with  more  or  less  quartz 
Sagvandite,  from  Norway,  is  a  granular  mixture  of  varieties  of  mag- 
nesite  and  enstatite  (MgSiO3)  containing  ferrous  iron. 

Amphibolites.  Many  of  the  hornblende-schists  or 
amphibolites,  previously  described  in  a  separate  section, 
are  the  result  of  the  transformation  of  impure  limestones 
and  dolomites  into  metamorphic  rocks.  This  has  been 
already  discussed,  but  it  should  be  again  mentioned  here, 
because  the  amphibolites,  made  in  this  way,  form  one  of 
the  most  important  members  of  the  lime  and  magnesia 
series  of  carbonate-silicate  rocks  described  above. 

Occurrence  of  Minerals  and  Ores.  The  crystalline  marbles  and 
dolomites,  in  addition  to  the  minerals  mentioned  above,  not  infre- 
quently, owing  to  local  causes,  contain  a  great  variety  of  others. 
Thus  at  Franklin,  New  Jersey,  owing  to  the  presence  of  zinc  and 
manganese,  a  number  of  minerals  containing  these  metals  have  been 


392  ROCKS  AND   ROCK  MINERALS 

produced,  forming  useful  ores.  Ore-bodies  are  mostly  developed  in 
these  rocks,  however,  by  contact  metamorphism,  but  in  some  cases 
the  minerals  developed  by  regional  metamorphism  are  of  such  a 
character,  and  in  such  quantity,  that  they  may  be  usefully  exploited. 
Many  of  the  famous  mineral  localities,  specimens  from  which  are 
commonly  seen  in  collections,  are  in  these  rocks.  The  minerals 
thus  found  embedded  as  crystals  in  calcite  and  dolomite,  are  apt  to 
have  the  angles  between  the  faces  more  or  less  rounded,  and  to  be 
veined  with  calcite  in  their  cracks. 

SERPENTINE. 

General  Properties.  No  close  distinction  between  ser- 
pentine, as  it  has  been  described  as  a  mineral,  and  serpen- 
tine as  a  rock  can  be  made.  As  a  mineral  the  chemically 
pure  substance  was  considered,  but  serpentine  as  a  rock 
is  generally  more  or  less  impure  from  the  presence  of 
other  minerals  which  are  mixed  with  it.  Serpentine 
rocks  are  generally  compact,  of  a  dull  to  waxy  luster,  and 
of  a  smooth  to  splintery  fracture.  If  tolerably  pure  they 
are  soft  and  can  be  cut  by  the  knife,  but  they  are  some- 
times saturated  by  deposited  silica,  which  makes  them 
much  harder.  The  general  color  is  green,  characteristically 
a  yellowish-green;  but  sometimes  yellow,  yellow-brown, 
reddish-brown,  and  dark  green  to  black.  On  smooth  sur- 
faces the  rock  has  a  somewhat  greasy  feel,  recalling  talc- 
schists,  from  which  it  is,  however,  readily  distinguished 
by  its  superior  hardness.  Talc  leaves  its  mark  on  cloth, 
while  serpentine  does  not.  The  yellow-green  color  re- 
sembles also  that  of  epidote  rocks,  but  here  again  the 
superior  hardness  of  the  epidote  serves  as  a  distinction. 

Associated  Minerals.  Other  minerals  which  may  accom- 
pany the  serpentine,  and  which  may  at  times  be  seen  in  it, 
are  remains  of  the  magnesia  silicates  from  which  it  has 
been  formed,  olivine,  pyroxene,  and  hornblende.  Metallic- 
looking  specks  or  crystals  of  ores  are  common,  magnetite, 
chromite,  etc.  In  some  varieties  garnet  occurs,  chiefly 
pyrppe,  and  that  which  is  used  for  gems  comes  in  large 
part  from  a  serpentine  in  Bohemia.  In  the  Ural  Mountains 


DESCRIPTION  OF  METAMORPHIC  ROCKS 


393 


serpentine  is  the  source  of  platinum,  and  in  other  places  of 
nickel  ores.  Serpentine  is  apt  to  be  accompanied  by 
other  secondary  minerals,  by  chlorite  (sometimes  the  pur- 
ple-red variety  kammererite  containing  chromium),  by 
talc,  and  by  magnesium  carbonates,  magnesite,  MgCOs,  and 
breunnerite,  MgFeCOs,  etc.  Serpentine  rocks  are  usually 
massive  but  sometimes  schistose,  serpentine-schist.  Not 
infrequently  they  are  seamed  by  veins  of  the  finely 
fibrous  variety  of  the  mineral  called  chrysotile,  which 
has  the  structure  of  asbestus  and  is  often  so  called. 

Chemical  Composition.  The  chemical  composition  of 
serpentine  rocks  approaches  that  of  the  pure  mineral,  but 
generally  differs  somewhat  on  account  of  the  other  minerals 
present.  This  is  seen  in  the  appended  analyses: 


SiO2 

AI203 

Cr2C>3 

Fe2O3' 

FeO 

NiO 

Mg 

CaO 

H20 

COa 

XyO 

Total. 

I.. 

40.4 

1.9 

0.3 

2.8 

4.3 

0.5 

36.0 

0.7 

10.7 

1.4 

0.6 

99.6 

II. 

36.6 

1.0 

0.3 

7.3 

0.4 

0.3 

40.3 

0.1 

13.4 

— 

0.4 

100.1 

Ill 

38.6 

1.3 

0.5 

5.6 

2.2 

0.1 

39.1 

0.9 

11.3 

0.5 

0.2 

100.3 

IV 

36.9 

1.4 

— 

6.9 

4.0 

— 

36.0 

1.4 

13.1 

— 

— 

99.7 

V. 

42.3 

0.8 

— 

_ 

2.6 

— 

40.3 

1.3 

12.5 

— 

0.5 

100.3 

VI 

44.1 

— 

— 

— 

— 

•— 

43.0 

~ 

12.9 

— 

~~ 

100.0 

I,  Serpentine,  dark-green,  Rowe,  Massachusetts;  II,  Serpentine, 
from  pyroxenite  dike,  Mount  Diablo,  California;  III,  Serpentine, 
Iron  Mountain,  Oregon;  IV,  Serpentine,  from  hornblende-schist, 
Vosges  Mountains,  Germany;  V,  Serpentine,  white,  selected  pure 
mineral,  Brewsters,  New  York;  VI,  Theoretical  composition  of  pure 
mineral,  H4Mg3Si20,. 

The  presence  of  small  quantities  of  nickel  and  chrome 
oxides  is  a  very  common  feature. 

Origin.  Serpentine  rocks  are  secondary  in  nature, 
being  formed  when  previously  existent  rocks,  consisting 
wholly  or  chiefly  of  magnesian  silicates,  are  exposed  to 
the  processes  at  work  in  the  zone  of  hydration.  Their 
origin  may  thus  be  twofold:  they  may  be  formed  from 


394  ROCKS  AND   ROCK  MINERALS 

igneous  rocks,  such  as  peridotite,  dunite,  etc.;  or  when 
amphibolites  or  hornblende-schists,  which  have  been 
made  from  sediments  in  the  zone  of  constructive  meta- 
morphism,  are  brought  by  erosion  into  the  zone  of  hydra- 
tion,  they  may  be  converted  into  serpentines.  Thus  the 
origin  of  the  material  may  be  igneous  or  sedimentary, 
but,  whereas  the  igneous  rocks  pass  directly  into  serpen- 
tine, the  sedimentary  ones  first  pass  through  an  interme- 
diate metamorphic  stage  (hornblende-schists,  etc.),  and 
are  then  converted.  In  this  connection  what  has  been 
said  elsewhere  concerning  the  alteration  of  the  peridotites 
and  allied  rocks  should  be  read  This  also  explains  in 
part  at  least  the  origin  of  the  chromium  and  nickel.  No 
formula  can  be  given  for  the  recognition  of  which  origin 
a  serpentine  has  had;  the  geologic  mode  of  occurrence  and 
relation  to  other  rock  masses  is  often  a  help,  while  the 
presence  of  nickel  and  chromium,  — •  substances  to  be 
expected  in  igneous,  but  not  in  sedimentary  rocks,  —  if 
it  can  be  shown,  is  very  significant. 

Occurrence.  Serpentine  is  a  common  rock,  and,  while 
it  rarely  forms  large  masses  or  covers  extensive  areas,  it  is 
widely  distributed  over  the  world.  In  the  form  of  layers, 
lenticular  masses,  etc.,  it  is  common  in  metamorphic 
regions  from  the  alteration  of  both  igneous  and  meta- 
morphic rocks,  and  it  thus  occurs  in  eastern  Canada, 
New  England,  New  York,  Pennsylvania,  Maryland,  Cali- 
fornia, Oregon,  and  other  states;  in  southern  England, 
Germany,  the  Alps  and  various  other  places.  It  also 
occurs  in  non-metamorphic  sedimentary  areas  due  to  the 
conversion  of  igneous  rocks  which  have  penetrated  the 
strata,  as  in  places  in  Quebec,  New  Brunswick,  New  York 
state,  etc. 

Alteration*  Uses.  Serpentine  shows  great  resistance  to 
the  action  of  the  weathering  agencies  at  the  surface,  but 
eventually  breaks  down  into  a  mixture  of  carbonates  and 
silica,  mixed  with  ferruginous  matter.  The  soils  thus 
formed,  on  account  of  the  lack  of  alkalies  and  lime,  are 


DESCRIPTION  OF  METAMORPHIC  ROCKS         395 

extremely  barren,  and  often  little  or  no  vegetation  grows 
upon  them. 

On  account  of  its  beautiful  coloring,  serpentine  has 
been  largely  quarried  for  use  as  an  ornamental  stone,  being 
used  for  interior  purposes  much  as  highly  colored  marbles 
are.  It  is  sometimes  employed  for  the  same  objects 
for  which  soapstone  is  used;  in  many  cases  its  softness  is 
an  objection  to  its  employment.  In  some  places  the 
seams  of  fibrous  chrysolite  which  it  contains  are  mined  for 
use  as  asbestus.  Its  value  as  a  source  of  ores  of  nickel, 
chromium,  etc.,  has  been  already  commented  upon,  and  is 
further  mentioned  under  peridotites  and  allied  rocks. 

IRON  OXIDE  ROCKS. 

Itabirite.  This  rock  is  composed  chiefly  of  micaceous 
hematite  and  quartz.  The  micaceous  hematite,  or  "  spec- 
ular iron  ore  "  as  it  is  often  called,  is  in  very  thin  tablets 
or  leaves  of  irregular  outline,  while  the  quartz  is  in  aggre- 
gates of  grains.  It  much  resembles  mica-schist,  and  if  one 
were  to  imagine  the  mica  of  such  a  schist  replaced  by  a 
substance  of  mica-like  thinness  but  with  the  metallic 
luster  of  polished  iron,  he  would  have  a  good  idea  of  the 
appearance  of  this  rock.  Micaceous  hematite  is  indeed 
of  not  infrequent  occurrence  in  genuine  mica-schists, 
and  by  its  increase  transition  forms  to  itabirite  are  pro- 
duced. Also,  just  as  the  relative  quantities  of  quartz 
and  mica  vary  in  different  layers  of  mica-schist,  so  do  the 
micaceous  hematite  and  quartz  vary  in  itabirite;  thus  there 
are  layers  poor  in  quartz,  and  others  quite  rich  in  it,  of  very 
variable  thickness.  In  addition  to  the  mica,  magnetite, 
pyrite,  talc,  garnet,  and  others  may  occur  as  perceptible 
accessory  minerals.  The  rock  is  generally  granular  to 
fine  granular;  very  schistose;  of  a  dark  color  on  the  cross- 
fracture,  and  exhibits  on  the  chief  fracture  the  shining 
steel-like  luster  of  the  specular  iron  ore.  Sometimes  the 
amount  of  the  iron  mineral  is  so  great  as  to  practically 


396  ROCKS  AND  ROCK  MINERALS 

conceal  the  quartz.  Itabirite  forms  extensive  areas  in 
Brazil  and  on  the  Gold  Coast  of  Africa,  and  in  these  places 
carries  native  gold.  It  also  occurs  in  North  and  South 
Carolina,  in  Canada,  Norway,  Germany,  etc.  It  has 
probably  been  formed  by  the  metamorphism  of  sandstones 
and  shales  rich  in  deposited  ferruginous  matter,  limonite, 
etc. 

Jaspilite  is  a  name  given  to  somewhat  similar  rocks 
which  consist  of  layers  of  red  chert  and  hematite.  They 
occur  in  the  Lake  Superior  Region.  See  page  297. 

Magnetite  Bock.  This  is  a  compact  to  granular  aggre- 
gate of  grains  of  magnetite;  dark-colored  to  black,  and 
heavy.  The  properties  are  those  described  under  the 
mineral.  Hematite  is  very  commonly  mixed  with  it,  and  a 
variety  of  other  minerals,  such  at  ilmenite,  pyrite,  quartz, 
calcite,  garnet,  etc.,  according  to  the  mode  of  occurrence. 
The  origin  of  magnetite  rock  is  various;  thus  it  may  occur 
as  masses  included  in,  or  associated  with,  igneous  rocks, 
and  is  then  regarded  as  a  differentiated  phase  of  such  rocks, 
as  mentioned  under  them,  and  in  this  case  the  associated 
minerals  vary  with  the  kind  of  rock,  as  nephelite  and  augite, 
when  with  nephelite-syenite  (Arkansas,  Brazil,  Sweden); 
olivine,  pyroxene,  lime-soda  feldspar  when  with  gabbros 
(Adirondacks,  Sweden,  Canada,  Colorado,  etc.).  In  other 
cases  it  occurs  as  a  contact  formation  where  igneous  rocks 
have  metamorphosed  beds  of  limonite,  siderite,  etc. 
Finally  it  occurs  in  regional  metamorphosed  areas,  in  the 
form  of  layers  and  lenses,  in  the  midst  of  gneisses  and 
schists,  and  often  associated  with  metamorphosed  lime- 
stones and  dolomites.  It  then  often  contains  carbonates 
of  lime  and  magnesia,  as  well  as  the  more  common  of  the 
silicate  minerals  described  as  associates  of  marble,  such 
as  garnet,  pyroxene,  hornblende,  etc.  It  is  probably  due 
to  the  metamorphism  of  beds  of  impure  limonite,  clay- 
ironstone,  etc.  Deposits  of  magnetite  rock  occur  in  many 
places  in  the  United  States  and  Canada,  in  Scandinavia, 
Germany,  the  Ural  Mountains,  etc.,  and  are  of  great 


DESCRIPTION    OF    METAMORPHIC    ROCKS       397 

importance  as  sources  of  iron  ore.  Those  which  are 
situated  in  genetic  connection  with  igneous  rocks  are, 
however,  generally  useless  on  account  of  the  presence  of 
ilmenite,  titanic  iron  ore,  which  prevents  their  being 
profitably  smelted. 

Emery.  This  is  a  granular  rock  of  a  dark-gray  to  black 
color,  consisting  mainly  of  grains  of  gray  or  bluish  corun- 
dum, often  mixed  with  magnetite,  and  associated  with 
other  minerals.  It  is  sometimes  quite  schistose.  It  is 
easily  told  by  its  weight  and  excessive  hardness  (corundum 
=  9).  It  occurs,  as  layers  of  relatively  small  volume  in 
the  crystalline  schists,  in  Asia  Minor,  Island  of  Naxos, 
Germany,  Massachusetts,  etc.  Its  use  as  an  abrasive 
material,  on  account  of  the  corundum  it  contains,  is  well 
known. 


CHAPTER  XII. 
THE  DETERMINATION  OF  ROCKS. 

THE  determination  and  classification  of  rocks  presents 
itself  as  a  problem,  whose  difficulty  depends  on  what  is 
sought  to  be  done,  and  the  means  at  command  for  carrying 
it  out.  It  is  obvious  that  the  fine  distinctions  made  by 
petrographers  among  rocks,  especially  the  igneous  ones, 
cannot  be  carried  into  ordinary  practice,  unless  the  same 
methods  for  the  study  of  rocks  —  the  use  of  the  micro- 
scope on  sections  ground  thin  and  chemical  analyses  —  are 
employed  which  they  use.  This,  of  course,  cannot  be  ordi- 
narily done,  and  we  are  thus  limited  to  the  means  of 
observation  which  have  been  used  in  this  work,  and  to 
simple  classifications  and  the  limited  number  of  kinds 
which  they  afford.  This  has  already  been  commented 
upon,  in  discussing  the  classification  of  the  igneous  rocks, 
and  need  not  be  repeated. 

Bock  Characters  used  in  Determination.  The  characters 
of  rocks  which  may  be  used  for  their  megascopic  determi- 
nation are  of  two  kinds,  mineral  and  general.  By  the 
mineral  characters  it  is  meant,  that  if  the  rock  is  composed 
wholly  or  in  part  of  mineral  grains,  which  are  large  enough 
to  be  distinctly  seen  with  the  eye  or  lens,  and  which  may  be, 
if  necessary,  handled  and  tested,  then  the  determination 
may  proceed  along  the  line  of  a  study  of  the  minerals,  their 
kinds,  relative  abundance,  and  relation  to  each  other 
(rock-texture) .  In  this  case  there  is  no  essential  difference 
between  the  microscopic  and  megascopic  study  of  rocks; 
one  can  accomplish,  in  the  main,  on  the  fractured  surface 
of  a  coarse  rock,  what  the  microscope  does  on  the  thin 
section  of  a  compact  one.  The  individual  minerals  may 

398 


THE  DETERMINATION  OF  ROCKS  399 

be  studied  and  tested  according  to  the  methods  given  in 
Chapter  V;  if  in  the  field,  the  simple  tests  of  Table  No.  I 
may  be  used;  if  the  conveniences  of  a  laboratory  are  at 
hand,  the  more  complete  one,  Table  No.  II  can  be  employed. 
If  it  has  been  already  determined,  perhaps  in  the  field, 
whether  the  rock  is  igneous,  sedimentary,  or  metamorphic, 
its  place  can  then  be  usually  very  quickly  settled.  Even 
if  all  the  different  kinds  of  minerals  cannot  be  told,  the 
determination  of  one  or  more  will  generally  be  of  service. 

The  general  characters  are  those  which  are  resultant 
from  the  combination  of  mineral  grains;  they  might  be 
termed  composite  features  of  rocks.  They  include  color, 
structure,*  texture,  fracture,  hardness,  and  specific  gravity. 
Of  these  the  specific  gravity  is  of  the  least  general  applica- 
bility, because  it  requires  a  special  apparatus  to  determine 
it.  The  reaction  of  the  rock  with  acids  is  also  at  times 
extremely  useful  as  a  test,  and  may  be  added  to  the  list. 
These  general  characters  are  so  useful  that  they  deserve 
some  separate  mention  in  regard  to  their  employment  in 
rock  determination. 

Color.  The  rock  color  is  the  general  resultant  of  those 
of  the  combined  mineral  grains.  Certain  general  conclu- 
sions may  be  drawn  from  the  color  of  a  rock;  thus  if  it  is 
pure  white  or  nearly  so,  it  is  certain  that  compounds  of 
iron  are  either  wanting  in  it,  or  are  only  present  in  traces, 
and  in  general  the  rock  is  either  a  sandstone,  quartzite, 
limestone-marble,  gypsum,  or  a  nearly  feldspathic  igneous 
rock,  such  as  anorthosite,  aplite,  syenite,  or  felsite.  Red, 
brown,  and  green  colors  indicate  the  presence  of  iron  com- 
pounds ;  black  or  stone  gray  may  also,  but  in  a  sedimentary 
rock,  these  colors  may  indicate  carbonaceous  material. 

Structure.  If  the  rock  has  a  pronounced  structure  of 
some  kind,  it  may  be  of  great  assistance  in  determining 
the  general  class  to  which  it  belongs,  and  this  may  be  of 
especial  assistance  if  the  geological  relations  of  the  rock 

*  The  difference  between  the  use  of  "  structure  "  and  "  texture  " 
has  been  already  explained,  p.  158. 


400  ROCKS  AND  ROCK  MINERALS 

mass  cannot  be  determined.  Thus  if  a  rock  mass  possesses  a 
pronounced  columnar,  or  a  highly  vesicular,  or  an  amygda- 
loidal  structure,  it  is  almost  certainly  of  igneous  origin; 
if  a  laminated  or  banded  structure,  it  is  probably  sedimen- 
tary; but  this  cannot  be  definitely  relied  on,  because 
igneous  rocks,  especially  lavas,  may  assume  a  banded 
structure  by  flowage,  while  metamorphic  rocks  may 
acquire  it  by  shearing  movement.  The  oolitic  structure 
indicates  a  sedimentary  rock.  In  general,  structure  must 
be  considered  in  connection  with  texture  and  other  prop- 
erties. 

Texture.  Certain  textures  are  of  definite  assistance  in 
determining  the  family  to  which  a  rock  belongs.  Thus 
the  glassy  texture  is  definite  proof  that  the  rock  is  of 
igneous  origin;  a  porphyritic  texture  shows  the  same 
thing,  especially  if  the  phenocrysts  are  well  crystallized, 
and  of  quartz,  or  feldspar,  or  both.  Metamorphic  rocks  also 
contain  at  times  well  crystallized  minerals,  which  are 
similar  to  phenocrysts,  as,  for  instance,  garnet  and  stauro- 
lite,  but  in  general  they  also  possess  at  the  same  time  a 
well  foliated  structure,  which  helps  to  distinguish  them. 
Sedimentary  rocks  do  not  exhibit  this  texture.  The  mere 
contrast  in  color  of  a  few  dark  mineral  grains  among  many 
lighter  ones  must  not  be  mistaken  for  a  porphyritic 
texture.  In  general,  a  hard,  firm,  highly  crystalline  tex- 
ture, alike  in  appearances  in  all  directions  through  the  rock, 
is  indicative  of  an  igneous  origin;  but  there  are  many 
exceptions  to  this  rule,  as  shown  in  various  marbles  and 
quartzites.  If  a  rock  has  a  highly  crystalline  texture, 
and  at  the  same  time  a  foliated  structure,  it  is  probably 
metamorphic. 

Hardness.  This  character,  which  can  readily  be  tested 
in  a  rough  way  in  the  field,  is  very  useful  in  distinguish- 
ing between  certain  classes  of  rocks.  Thus  very  fine- 
grained compact  sandstones  (or  quartzites),  limestones,  and 
dense  igneous  rocks  often  look  much  alike  in  specimens. 
A  simple  test  of  hardness  with  the  knife-point  will  at  once 


THE  DETERMINATION  OF  ROCKS  401 

distinguish  between  the  limestones  (carbonate  rocks,  soft) 
and  the  others  mentioned  (silicate,  or  silica  rocks,  hard). 
If  the  rock  is  not  very  firm,  care  must  be  taken  not  to  con- 
fuse the  mere  breaking  down  or  crushing  of  the  rock  fabric 
with  actual  scratching  of  its  component  minerals.  If  the 
rock  itself  is  used  to  scratch  with,  care  must  also  be  taken 
to  test  a  number  of  corners,  or  edges,  so  that  some  single 
grain,  harder  than  the  average,  may  not  produce  a  false 
impression  of  the  average  hardness. 

Fracture.  This  is  of  less  importance  than  the  foregoing 
characters,  but  yet  in  some  ways  is  of  value.  Most  rocks 
which  are  firm  and  solid  enough  to  have  a  distinct  fracture 
exhibit  a  more  or  less  rough,  hackly  one.  Those  which 
are  fine-grained,  or  dense  and  compact,  and  which  contain 
a  large  amount  of  silica,  or  are  wholly  composed  of  it, 
such  as  felsites,  quartzites,  flint,  etc.,  possess  a  more  or  less 
distinct  conchoidal  fracture,  and  the  surface  may  be 
splintery.  Some  dense  limestones  also  have  a  splintery 
fracture,  and  may  even  approach  the  conchoidal.  Natural 
glasses,  such  as  compact  obsidian,  have  a  beautiful  con- 
choidal fracture. 

Specific  Gravity.  This  property  is  of  much  greater 
value  in  the  determination  of  minerals  than  of  rocks.  It 
cannot  of  course  be  used  in  the  field,  as  it  requires  definite 
apparatus  to  determine  it,  as  described  in  Chapter  III, 
under  minerals;  nevertheless,  even  in  the  field,  a  rough 
distinction  may  be  made  between  light  and  heavy  rocks, 
by  weighing  them  in  the  hand.  Rocks  that  are  dark- 
colored  and  very  heavy,  in  general,  are  composed  largely, 
or  chiefly,  of  iron-bearing  minerals,  and  are  apt  to  be  of 
igneous,  or  of  metamorphic  origin. 

Treatment  with  Acid.  This  is  particularly  useful  in 
distinguishing  the  carbonate  from  the  silicate  rocks. 
The  method  of  treatment  has  been  fully  described  in 
Chapter  V.  and  need  not  be  repeated  here.  If  necessary 
almost  any  acid  may  be  used,  such  as  vinegar  (acetic 
acid),  or  lemon  juice.  For  field  use  a  few  crystals  of 


402  ROCKS  AND  ROCK  MINERALS 

citric  acid  powdered  up  may  be  conveniently  carried,  and 
dissolved,  when  needed,  in  a  little  water;  the  test  for 
effervescence  can  thus  be  readily  made.  The  test  for 
gelatinization,  as  also  described  in  Chapter  V,  is  also  very 
useful  in  determining  the  nephelite  syenites  from  other 
syenites  and  from  granites,  and  also  its  effusive  represen- 
tative, the  phonolite  variety  from  the  other  felsites.  It 
should  be  remembered  that  olivine,  which,  however,  chiefly 
occurs  in  the  dark  ferromagnesian  rocks,  gabbros,  perido- 
tites  and  basalts,  also  gives  this  gelatinization  test. 

Determination  in  the  Field.  The  best  method  of  deter- 
mining the  family  to  which  a  rock  belongs,  that  is,  whether 
it  is  sedimentary,  igneous,  or  metamorphic,  is  to  study  its 
characters  in  the  place  in  which  it  occurs,  and  its  relation 
to  other  rock  masses.  For  these  features,  and  the  larger 
ones  of  its  structure,  may  be  very  apparent  in  the  field, 
while  a  simple  hand-specimen  may  entirely  fail  to  show 
them.  The  structure  of  a  granite-gneiss,  for  instance, 
may  be  very  clear  on  the  surface  of  a  field  exposure,  and 
be  quite  inappreciable  in  a  small  specimen.  It  is  not 
necessary  to  give  here  the  characters  and  relations  by 
which  the  class  may  be  determined;  this  is  geological 
rather  than  petrographical,  and  has  been  sufficiently  com- 
mented upon  in  Chapters  VI,  VIII  and  X.  If  the  family 
has  been  determined  in  the  field,  and  the  rock  is  coarse- 
textured,  so  that  the  mineral  grains  can  be  seen,  and  if 
necessary  handled,  Table  I  (p.  124)  of  Chapter  V  may 
be  used  for  their  identification,  and  by  then  referring  to 
the  classification  of  the  appropriate  family,  its  place  in 
general  can  be  readily  determined. 

Table  for  Rock  Determination.  Appended  to  this  chap- 
ter is  a  table  which  may  be  used  for  the  determination  of 
the  more  important  kinds  of  rocks.  It  is  based  essentially 
on  the  one  given  by  Geikie,  in  his  Textbook  of  Geology, 
which  has,  however,  been  considerably  modified,  and 
extended  to  meet  the  needs  of  this  work.  As  the  tests 
which  it  demands  are  very  simple,  consisting  for  the  most 


THE  DETERMINATION  OF  ROCKS  403 

part  of  those  relating  to  hardness  and  effervescence  with 
acid,  it  may  be  readily  used,  even  in  the  field.  It  must  be 
remembered,  however,  that  a  table  of  this  nature  can  be 
only  quite  general  in  character,  and  applicable  to  rocks  of 
well-defined  types.  Rock  kinds  grade  into  one  another  in 
so  many  ways,  as  has  been  described  in  a  number  of  places 
in  this  work,  that  not  only  the  student,  but  even  the 
experienced  geologist,  will  sometimes  be  puzzled  as  to 
the  proper  designation  a  particular  type  should  receive. 
But  if  this  fact  is  borne  in  mind,  it  is  believed  the  table 
will  prove  useful  in  aiding  one  to  classify  the  common 
rocks. 


404  ROCKS  AND  ROCK  MINERALS 


TABLE  FOR  DETERMINING  THE  COMMON  ROCKS. 

The  newly  fractured  surface  of  the  unaltered  rock  shows 
one  of  the  following  cases: 

a.  It  is  wholly  or  partly  glassy.     See  A  beyond. 

6.  Not  glassy;  of  a  dull,  even  appearance  or  stony;  with- 
out particular  texture,  or  so  compact  that  the  indi- 
vidual grains  cannot  be  seen  or  recognized.  See  B. 

c.  Distinctly  grained  and  crystalline;  the  grains  can  be 

seen  and  determined.    See  C. 

d.  Has  a  distinctly   foliated  or    gneissoid    structure. 

SeeD. 

e.  Has  a  clearly  fragmental  composition.    See  E. 

A.  Wholly  or  partly  glassy. 

1.  Wholly  of  glass ;  solid ;  strong  vitreous  luster.    Obsidian,  p.  262. 

2.  Wholly  of  glass;  solid;  resinous  or  dull  pitchy  luster.     Pitch~ 
stone,  p.  265  (Obsidian  and  pitchstone  may  contain  spherulites.) 

3.  Wholly  of  glass,  but  cellular  or  froth-like.     Pumice,  p.  266. 

4.  Of  glass,  but  enamel-like,  and  composed  of  small,  concentric 
spheroids.     Perlite,  p.  265. 

5.  Partly  of  glass  and  partly  of  distinct,  embedded  crystals. 
Vitrophyre,  p.  267. 

(The  above  forms  are  generally  associated  with,  or  pass  into, 
felsite  lavas.) 

6.  Glass  associated  with,  or  passing  into,  basalt;  rare.     Tachylite, 
p.  268. 

B.  Compact  close-grained,  and  dull  or  stony;  not  glassy. 

a.  Very  soft;  can  be  scratched  with  the  finger-nail. 

1.  Has  a  strong  earthy  or  clay  odor  when  breathed  upon;  rubbed 
strongly  between  the  fingers  has  ultimately  a  smooth,  greasy  feeling; 
does  not  effervesce  with  acids.     Various  colors.     Clay,  p.  327. 

2.  Friable;  crumbling;  soils  the  fingers;  little  or  no  clay  odor; 
lively  effervescence  with  acids;  color  white  or  light  yellowish,  etc. 
Chalk,  p.  310  or  perhaps  marl,  p.  313.     (Marl  may  give  a  good  clay 
odor.) 


THE  DETERMINATION  OF  ROCKS  405 

TABLE  FOR  DETERMINING  THE   COMMON   ROCKS 

Continued. 

3.  General  characters  as  in  2,  but  does  not  effervesce  with  acids. 
Diatomaceous  earth,  p.  298. 

4.  Harder,  more  compact  than  1,  2,  and  3.    No  clay  odor;  does 
not  effervesce;  composed  of  a  mineral  with  a  good  cleavage;  some- 
tunes  fibrous;  occurs  in  beds  or  veins.     Gypsum,  p.  293. 

5.  White  to  green,  or  gray;  does  not  effervesce;  no  clay  odor; 
mass  has  a  soft,  greasy  feel ;  is  often  foliated  or  shows  a  micaceous 
cleavage;  folia  inelastic;  marks  cloth.     Talc-rock,  p.  374. 

6.  Not  scratched  by  the  nail,  but  easily  scratched  or  cut 
with  the  knife. 

1.  Composed  of  excessively  fine,  almost  imperceptible  particles; 
dull,  even  appearance;  gives  clay  odor  when  breathed  on;  no  effer- 
vescence or  but  feeble ;  has  a  laminated  or  stratified  structure  and 
usually  breaks  easily  into  chippy  flakes ;  generally  gray,  but  often  red, 
yellow,  brown,  bluish,  or  black.     Shale,  p.  327. 

2.  No  clay  odor,  or  but  feeble ;  brisk  effervescence  with  acid;  white 
streak;  commonly  gray;  sometimes  white  to  brown  or  black.     Lime- 
stone, p.  304. 

3.  As  in  2,  but  feeble  effervescence  in  acid,  which  becomes  brisk 
when  the  acid  is  heated;  generally  white,  yellowish  or  pale  brown. 
Dolomite,  p.  307. 

4.  Pale  to  dark  green  or  black,  sometimes  reddish;  soapy  or 
greasy  feel;  translucent  on  thin  edges;  waxy  or  oily  appearing; 
subconchoidal  or  splintery  fracture;  no  effervescence.     Serpentine, 
p.  392. 

c.  Not  scratched  or  cut  with  the  knife;  scratches  glass; 
does  not  effervesce  with  acid. 

1.  Various  colors,  white  to  red  or  purple,  brown  to  dark  gray; 
often  gives  a  clay  odor;  frequently  shows  banded  flow  structure. 
Felsite,  p.  248. 

2.  Very  hafd;  any  corner  or  angle  scratches  feldspar;  no  clay 
odor;  scratches  steel  readily;  light  colors  to  brown  or  black;  pro- 
nounced  conchoidal   fracture;  glimmering  horny   appearance.     A 
siliceous  rock;  either  flint,  p.  297,  or  perhaps  the  rhyolite  variety  of 
felsite,  p.  249. 


406  ROCKS  AND  ROCK  MINERALS 

TABLE  FOR  DETERMINING  THE  COMMON  ROCKS 

Continued. 

3.  Not  so  hard  as  1  and  2.  Does  not  scratch  feldspar;  color 
black,  very  dark  gray  or  green;  is  heavy;  sometimes  shows  a  cellular 
or  slaggy  structure;  sometimes  contains  amygdules.  Basalt,  p.  254. 

C.  Distinctly  grained  and  crystalline;  grains  wholly  or  partly 
determinate. 

a.  Is  easily  scratched  with  the  knife. 

1.  Effervesces  briskly  with  acid.    Limestone  p.  304,  or  more  prob- 
ably marble,  p.  384. 

2.  Effervesces  briskly  only  when  the  powdered  rock  is  treated 
with  hot  acid.     Dolomite  marble,  p.  390. 

3.  Does  not  effervesce;  probably  granular  crystalline.     Gypsum, 
p.  293,  or  anhydrite,  p.  295. 

4.  Soluble,  with  distinct  saline  taste.     Rock-salt,  p.  295. 

b.  Hard;  cannot  be  scratched  with  the  knife,  or  scratches 
with  difficulty.    Silicate  rocks,  two  cases  arise,  x  and  y. 

x.  It  is  composed  of  grains  of  approximately  equal  size; 
i.e.,  it  is  even-granular,  like  common  granite.  See  X. 

y.  It  is  composed  of  larger,  distinct  crystals  embedded 
in  a  finer-grained  groundmass;  i.e.,  it  is  a  porphyry. 
SeeT. 

X.  An  even-granular,  massive,  silicate  rock.     See  p.  155. 

1.  Mainly  or  wholly  composed  of  quartz  and  feldspar.    Granite, 
p.  205.     See  also  aplite,  p.  214. 

2.  Mainly    or    wholly   composed    of   feldspar  without    quartz. 
Syenite,  p.  218.     See  also  nephelite  syenite,  p.  221  and  anorthosite, 
p.  224. 

3.  Composed  of  feldspar  and  a  dark  ferromagnesian  mineral ;  the 
latter  equals  or  exceeds  the  feldspar;  a,  the  dark  mineral  is  mostly  or 
wholly  hornblende.    Diorite,  p.  226;  b,  the  dark  mineral  is  mostly 
or  wholly  pyroxene.    Gabbro,  p.  229 ;  c,  the  dark  mineral  is  indeter- 
minable.    Dolerite,  p.  235. 

4.  Composed  entirely,  or  almost  entirely,  of  ferromagnesian  min- 
erals; generally  heavy  and  dark  green  to  black  (sometimes  yellowish, 
dunite).     Peridotite,  pyroxenite,  etc.,  see  p.  238. 


THE  DETERMINATION  OF  ROCKS  407 

TABLE  FOR  DETERMINING  THE  COMMON   ROCKS 
Continued. 

5.  Composed  of  grains  of  quartz ;  scratches  glass  or  feldspar  readily 
Sandstone,  p.  323,  or  quartzite  p.  366. 

6.  Much  less  commonly  than  the  above,  massive  silicate  rocks 
produced  by  metamorphism  may  occur  in  this  division.     There  are  a 
number  of  different  varieties,  depending  on  the  particular  mineral,  or 
minerals.     Epidote-rock,  garnet-rock,  etc.,  would  be  examples.     See 
contact-metamorphism,    pp.   180,    186,  and   carbonate-silicate    rocks, 
p.  384. 

Y.  A  porphyry  (see  p.  156),  composed  of  phenocrysts  and 
a  groundmass. 

1.  Phenocrysts  of  quartz  and  feldspar  and,  perhaps,  of  a  ferro- 
magnesian  mineral  in  a  groundmass  of  the  same.     Granite-porphyry, 
p.  243. 

2.  Phenocrysts  of  feldspar  (and  often  of  a  ferromagnesian  min- 
eral)  in  groundmass  of   predominant  feldspar.     Syenite-porphyry, 
p.  243. 

3.  Phenocrysts  of  ferromagnesian  minerals,  or  feldspar,  or  both,  in 
a  groundmass  of  feldspar  and  ferromagnesian  minerals;  feldspar 
phenocrysts  frequently  striated.     Diorite-porphyry,  p.  244. 

4.  Phenocrysts  of  quartz,  or  feldspar,  or  both,  and  sometimes  of 
ferromagnesian  minerals,  in  a  predominant  groundmass  of  light  color 
and  dense  feldspathic  aspect.     Felsite-porphyry,  p.  251. 

5.  Phenocrysts  of  feldspar,  or  of  a  ferromagnesian  mmeral,  or 
both,  in  a  dense,  dark  to  black,  and  heavy  groundmass.    BasaU- 
porphyry,  p.  254. 

D.  It  has  a  distinctly  foliated,  gneissoid,  or  slaty  structure. 

1.  It  contains  feldspar,  and  generally  quartz,  with  mica  (some- 
times hornblende).     Gneiss,  p.  351. 

2.  It  consists  mainly  or  largely  of  mica;  often  considerable  quartz 
is  present,  but  feldspar  is  absent,  or  indeterminable.     Frequently 
contains  crystals  of  dark  red  to  black  garnet,  more  rarely  staurolite, 
cyanite,  etc.     Mica-schist,  p.  361. 

3.  Medium  green,  dark  green  or  black;  consists  mostly  of  a  felted 
or  matted  mass  of  small,  to  very  fine  or  microscopic,  bladed,  or  needle- 
like  crystals  arranged  mostly,  in  one  general  direction,  which  promotes 
the  schistose  cleavage.     Other  minerals,  such  as  garnet,   may  be 
pKsen.t.Hornblende-8chist  or  amphibolite  (p.  379). 


408  ROCKS  AND  ROCK  MINERALS 

TABLE  FOR  DETERMINING  THE  COMMON  ROCKS 
Continued. 

4.  Very  compact,  or  dense  and  fissile,  splitting  easily  into  thin, 
more  or  less  tough,  ringing  slabs ;  usually  dark  gray,  or  green  to  black, 
but  sometimes  showing  other  colors.     Slate,  p.  369.     (Sometimes 
contains  large  crystals  of  staurolite,  andalusite,  etc.) 

5.  Very  fissile,  but  soft  to  the  feel ;  laminae  not  tough,  but  often 
brittle  or  crumbling;  pronounced  silky  luster  on  the  cleavage  face. 
Phyllite,  p.  372. 

6.  Soft,  greasy  feel;  marks  cloth;  easily  scratched  with  the  finger- 
nail ;  usually  whitish  to  light  gray,  or  green.     Talc-schist,  p.  374. 

7.  Smooth  feel;  soft;  green  to  dark  green;  glimmering  luster. 
Chlorite-schist,  p.  376. 

E.  Has  a  clearly  fragmental  composition;  is  seen  to  be  com- 
posed of  fragments  or  pebbles  of  other  rocks,  or  of  smaller 
angular  or  -rounded  mineral  fragments;  if  the  latter, 
frequently  shows  evidences  of  stratification. 

1.  The  pebbles  range  from  the  size  of  a  pea  up  and  are  rounded; 
quartz  ones  are  common;  they  are  embedded  in  more  or  less  of  a 
cement.     Conglomerate,  p.  320. 

2.  The  pebbles  are  angular  in  shape.     Breccia,  p.  321. 

3.  Composed  of  various-sized  angular  fragments  of  volcanic  rocks, 
such  as  felsite  and  felsite  porphyry,  of  bits  of  pumice,  or  cellular  lava, 
or  of  rounded,  vesicular,  volcanic  bombs,  etc.,  mixed  with  fine  com- 
pacted material  (volcanic  ash).     Volcanic  tuff  and  breccia,  p.  272. 

4.  Composed  of  more  or  less  angular,  but  sometimes  rounded 
grains,  in  size  from  that  of  a  pea  down;  the  grains  are  mostly,  or 
wholly,  composed  of  quartz,  and  scratch  feldspar.     Generally  some 
cement  is  present,  which,  if  the  rock  is  light  colored,  is  apt  to 
effervesce  with  acid  (lime  carbonate);  if  red  or  brown  does  not. 
Sandstone,  p.  323. 

5.  As  ,in  4  but  more  or  less  feldspar  is  also  [present  among  the 
quartz  grains.     Arkose,  p.  326. 


INDEX 


Actinolite,  61,  63. 
Adinole,  188. 
Adobe,  331. 
Aegirite,  55,  58. 
Aeolian  rocks,  275. 
Albite,  34. 

Alkalic  feldspar,  34,  35. 
Alumina,  test  for,  117. 
Amphibole,  60. 

"  determination  of,  66. 

Amphibolite,  379,  391. 
Amygdaloidal  structure,  159 
Amygdaloid,  basalt,  256. 
Analcite,  103. 
Andalusite,  76. 
Andesite,  250. 
Anhydrite,  mineral,  113. 
Anhydrite,  rock,  295. 
Anorthosite,  224. 
Anorthite,  34. 
Anthracite,  319. 
Apatite,  95. 
Aplite,  214. 
Arfvedsonite,  61,  63. 
Argillite,  369. 
Arkose,  326. 
Aschistic  rocks,  169. 
Ashes,  volcanic,  141. 
Associations  of  minerals,  29. 
Augite,  55,  57. 
Auqitophyre,  257. 
Average  rock,  composition,  18. 

Basalt,  basalt-porphyry,  254. 

"       amygdaloid,  256. 

"      quartz,  256. 
Bathylith,  139. 
Bauxite,  97. 
Biotite,  50,  51. 
Bituminous  coal,  318. 
Black-band  ore,  302. 
Bog  iron  ore,  301. 
Bombs,  volcanic,  141. 


Border   zones   in   igneous   rocks, 

165. 

Border  zones,  origin  of,  170. 
Boss,  138. 
Bostonite,  254. 
Breccia,  friction,  322. 

"        sedimentary,  321. 

"        volcanic,  140,  272. 

"  "       origin,  269. 

Brown  coal,  317. 
Brownstone,  326. 
Breunerite,  110. 
Buhrstone,  368. 

Calcite,  105. 
Calcium,  test  for,  119. 
Calcareous  tufa,  312. 
Camptonite,  257. 
Cancrinite,  48. 
Carbonates,  test  for,  115. 
Cementation,  zone  of,  338. 
Chalcedony,  86. 
Chalk,  310. 

Chemical  elements,  18. 
Chert,  297. 

Chlorine,  test  for,  121. 
Chlorite,  98. 
Chlorite-schist,  376. 
Chloritoid,  54. 
Chondrodite,  82. 
Chrysotile,  101. 
Cipolin,  390. 
Classification,  general,  6. 

"  fragmental  volcanic 

rocks,  271. 
Classification,  glassy  rocks,  261. 

"  igneous  rocks,   191, 

194,  203. 
Classification,  igneous  rocks,  table, 

195. 
Classification,  metamorphic  rocks, 

348. 
Classification,  stratified  rocks,  290. 


409 


410 


INDEX 


Clay,  96,  327,  278,  280. 
Clay  ironstone,  301. 
Cleavage  of  minerals,  26. 

effect  of,  28. 
Coal,  315. 

"     hard,  319. 

"     soft,  318. 
Color  of  minerals,  23. 

"      "  rocks,  399. 

"       "  sedimentary  rocks,  286. 
Columnar  structure,  162. 
Comagmatic  regions,  174. 
Complementary  dikes,  167. 
Conglomerate,  sedimentary,  320. 

"  volcanic,  322. 

Consanguinity  of  rocks,  173. 
Contact  metamorphism,  180. 

"  "  effect   on 

limestone,  187. 
Contact  metamorphism,  effect  on 

sandstone,  186. 
Contact  metamorphism,  effect  on 

shale,  slate,  188. 
Contact  metamorphism,  endomor- 

phic,  181. 
Contact    metamorphism,   exomor- 

phic,  183. 
Contact  metamorphism,  modes  of 

occurrence,  185. 
Contact  metamorphism,  pneumato- 

lytic,  189. 
Contact  metamorphism,  ore  bodies, 

190. 

Coquina,  311. 
Cortlandtite,  238. 
Corundum,  86. 
Corundum-syenite,  226. 
Crystals,  denned,  21. 

"         form  in  rocks,  22. 
"         twinning  of,  36. 
Cyanite,  78. 

Docile,  250. 

Decay  of  rocks,  276. 

Determination  of  minerals,  114. 

"          "         tables, 
122. 
Determination  of  rocks,  396. 

"       "  table, 

404. 
Diabase,  235. 


Diaschistic  rocks,  169. 
Diatomaceous  earth,  298. 
Differentiation,  164,  169. 
Dikes,  134. 

"      complementary,  167. 
Diopside,  55,  57. 
Diorite,  226. 

"        rock  relations,  229. 
Diorite-porphyry ,  244. 
Dolerite,  235. 

"         use  of  word,  198. 

"         alteration  of,  237. 
Dolerite-porphyry ,  244. 
Dolomite,  mineral,  108. 
Dolomite,  rock,  307. 

"  "      origin,  308. 

Dolomite-marble,  390. 
Dunite,  238,  240. 

Earth's  crust,  composition,  17. 

"        interior,  state  of,  14. 
Eclogite,  382. 
Elements,  geologically   important 

18. 

Emery,  397. 
Epidosite,  389. 
Epidote,  73. 

Eutaxitic  Structure,  164. 
Exotic  mineral  colors,  24. 
Extrusive  igneous  rocks,  139. 

Feldspars,  34. 

alteration  of,  44. 
"          cleavage  of,  40. 
"          color  of,  41. 

crystal  form  of,  35. 
determination  of,  46. 
twinning  of,  36. 
Feldspathoid  minerals,  47. 
Felsite,  felsite-porphyry,  248. 
"       sheared,  373. 
"        varieties  of,  249. 
Ferromagnesian  minerals,  146. 
Field  classification,  6. 
Flint,  86,  296,  297. 
Fluorine,  test  for,  121,  81. 
Fracture  of  minerals,  29. 
"         conchoidal,  29. 
"         of  rocks,  401. 
Fragmental  volcanics,  269. 
Freestone,  325. 


INDEX 


411 


Gabbro,  229. 

"       alteration  of,  232. 
"       iron  ores  in,  234. 
Garnets,  70. 
Garnet-rock,  389. 
Gelatinization  test,  115. 
Geyserite,  296,  298. 
Glassy  rocks,  260. 

"      alteration  of,  269. 
"  "      classification,  261. 

Glaucophane,  64. 
Glaucophane-schist,  383. 
Gneiss,  351. 

"       field  study  of,  358. 
"       inclusions  in,  356. 
"       texture  of,  353. 
"       varieties  of,  355. 
Granite,  205. 

complementary  dikes  of,  214. 
contact  of,  215. 
"       graphic,  212. 
"        orbicular,  211. 
"        pegmatites  in,  212. 
"        porphyritic,  207. 
"        weathering  of,  216. 
Granite-porphyry,  243. 
Granulite,  360. 
Gravel,  278,  279. 
Graywacke,  326. 
Greenstone,  229,  377. 
Greenstone-schist,  377,  383. 
Greensand-marl,  325. 
Grit,  325. 

Ground  mass  denned,  156. 
Gruss,  216. 

Gypsum,  mineral,  111. 
Gypsum,  rock,  293. 

Halite,  113,  295. 
Hammer,  geological,  11. 

"          trimming,  12. 
Hardness  of  minerals,  30. 

"  "  rocks,  400. 

Hauynite,  48. 
Hematite,  91,  302. 
Heulandite,  104. 
Hornblendes,  60. 
Hornblende-schist,  379. 
Hornblendite,  238. 
Hornfels,  188. 
Hornstone,  188,  297. 


Hydromica-schist,  373. 
Hypersthene,  55,  57. 

Ice,  20. 

Igneous  rocks,  132,  205. 

"       consanguinity      of, 
173. 
Igneous    rocks,    classification    of, 

191,  194,  203. 
Igneous    rocks,  crystallization  in, 

146. 
Igneous  rocks,  dense  types,  247. 

"  "       general  characters, 

132,  141. 

Igneous  rocks,  inclusions   in,  163. 
jointing  of,  161. 
minerals  of,  145. 
occurrence  of,  134. 
origin  of,  164. 
petrology  of,  132. 
post  intrusive  work 
of,  174. 

Igneous  rocks,  structure  of,  158. 
"  "      textures     of,     150, 

154. 
Igneous  rocks,  variation  of  minerals 

in,  142. 
Ilmenite,  90. 

Inclusions  in  granite,  213. 
"  gneiss,  356. 
"  igneous  rocks,  163. 
Injection  of  schists,  345. 
Intrusive  sheets,  135. 
Infusorial  earth,  298. 
Iron  ores,  88,  299,  395. 
Iron  oxide  rocks,  395. 
Iron,  test  for,  118. 
Itabirite,  395. 

Jadeite,  389. 
Jade,  389. 
Jasper,  86,  297. 
Jaspilite,  297,  396. 
Jet,  319. 
Jointing,  161. 

"         in  granite,  210. 

Kammererite,  99,  393. 
Kaolin,  96. 

"        from  feldspar,  44. 
Kimberlite,  242. 


412 


INDEX 


Labradorite,  35  (rock,  224). 
Laccoliths,  136. 

"       zoned,  166. 
Lamprophyre,  defined,  168. 
Lamprophyres,  257. 
Lapilli,  volcanic,  141. 
Laterite,  332. 
Lava  flows,  139. 
Lepidomelane,  52. 
Lepidolite,  52. 
Leucocratic  rocks,  167. 
Leucophyre,  251. 
Leucite,  49. 

"          rocks,  259. 
Lignite,  317. 

Lime-carbonate-silicate  rocks,  387. 
Limestone,  303. 

"         oolitic,  309. 
Lime,  test  for,  119. 
Limonite,  93,  301. 
Listw'dnite,  375,  391. 
Lithographic  stone,  306. 
Lithology  defined,  2. 
Lithophysae,  264. 
Loam,  332. 
Loess,  330. 
Lydianite,  297. 

Magmas,  134. 

"          composition  of,  141. 
"          variations  in,  142. 
Magnesia-silicate  rocks,  390. 
Magnesite,  110. 
Magnesium,  test  for,  119. 
Magnetite,  mineral,  89. 
Magnetite-rock,  396. 
Marble,  384. 
Marble,  onyx,  313. 
Marl,  313. 

"     greensand,  325. 
Megascopic,  defined,  7. 
Melanocratic  rocks,  168. 
Melaphyre,  255. 
Metadiorite,  229. 
Metamorphic  rocks,  333. 

"        age    of,  346. 
"  "        classification 

of,  348. 
Metamorphic    rocks,    composition 

of,  344. 

Metamorphic   rocks,   injection   of, 
345. 


Metamorphic    rocks,   minerals   of, 

339. 
Metamorphic  rocks,  occurrence  of, 

346. 

Metamorphic    rocks,    older   struc- 
tures in,  343. 

Metamorphic  rocks,  origin  of,  333. 
Metamorphic    rocks,    textures    of, 

340. 
Metamorphism,  333. 

"          agents  of,  335. 
"          constructive,  338. 
"          effect  of  depth,  337. 
"  "     "  heat,  336. 

"  "     "  liquids,  337. 

Miarolitic  structure,  159. 
Micas,  50. 
Mica-schist,  361. 
Mica-trap,  257,  215. 
Microcline,  36. 

Microscopical  petrography,  7. 
Minerals,  associations  of,  29. 
"         cleavage  of,  26. 
"         color  of,  23. 
"         exotic  color  of,  24. 
"         defined,  21. 
"         determination  of,  114. 
"         fracture  of,  29. 
"         hardness  of,  30. 
"         specific  gravity  of,  31. 
"         streak  of,  25. 
Mineralizers,  149. 
Minette,  237,  257. 
Muscovite,  50,  51. 

Natrolite,  103. 
Necks,  volcanic,  138. 
Nephelite,  47. 
Nephelite-syenite,  221. 
Nephrite,  390. 
Norite,  229. 
Noselite,  48. 
Novaculite,  297. 

Obsidian,  262. 

Occurrence  of  igneous  rocks,  134. 

Ocher,  328. 

Olivine.  67. 

"       nodules,  258. 

"        rock,  238,  240,  390. 
Onyx  marble,  313. 


LNDEX 


413 


Oolite,  iron,  303. 

"       limestone,  309. 

"       siliceous,  368. 
Opal,  86. 
Ophicalcite,  391. 
Order  of  crystallization,  146. 
Ore  bodies,  190,  391. 

"        formation  of,  170. 
Origin  of  igneous  rocks,  164. 

"       "  metamorphic  rocks,  333. 
Orthoclase,  34. 
Oxides,  important,  20. 

Paragonite,  51. 
Peat,  316. 
Pebbles,  279. 
Pegmatite  dikes,  175. 

"  "     origin,  178. 

Peridotite,  238. 

ores  in,  241. 
relation  to  gabbro,  240. 
Perlite,  265. 

Petrographic  provinces,  173. 
Petrography  defined,  2. 

microscopical,  7. 
Petrology  defined,  1,  2. 

"          history  of,  4. 

"          of  igneous  rocks,  132. 
Phenocrysts,  156,  245. 

"  pseudo,  342. 

Phlogopite,  52. 
Phonolite,  250. 
Phosphate  rock,  314. 
Phosphoric  acid,  test  for,  122. 
Phosphorite,  314. 
Phyttite,  372. 
Pitchstone,  265. 
Plagioclase  feldspar,  34,  35. 
Poikilitic  texture,  239. 
Porphyroid,  373. 
Porphyry,  156,  242. 

"          basalt,  254. 

"          classification  of,  195. 

"         diorite,  244. 

"         dolerite,  244. 

"          felsite,  251. 

"         granite,  243. 

"         labradorite,  257. 

"         syenite,  243. 
Post-intrusive  processes,  174. 
Potash,  test  for,  120. 
Pudding-stone,  321. 


Pumice,  266. 
Pyrite,  94. 
Pyroxene,  55,  57. 

alteration  of,  58. 
"         determination  of,  00. 
Pyroxene-rock,  389. 
Pyroxenite,  238. 

Quantitative  classification,  203. 
Quartz,  83. 
Quartzite,  366. 

"  oolitic,  368. 

Rhyolite,  250. 

Rock  salt,  113,  295. 

Rocks  defined,  3. 

"      determination  of,  398. 
"      general  classification  of,  6 
"      table  to  determine,  405. 

Sagvandite,  391. 
Salic  defined,  146. 
Sand,  278,  280. 
Sandstone,  323. 
Scale  of  hardness,  30. 
Schist,  talc,  374. 

"      mica,  361. 

"       hornblende,  379.  t 

hydro-mica,  373. 

"       greenstone,  377. 

"       glaucophane,  383. 

"      chlorite,  376. 
Schistose  texture,  340. 
Schlieren,  164. 
Schorl,  78. 
Scoria,  266. 
Selenite,  111. 
Sericite,  54. 
Serpentine,  100. 
Serpentine,  rock,  392. 
Shale,  327. 

"      alum,  329. 
Siderite,  110,  301. 
Silica,  test  for,  115. 
Sillimanite,  78. 
Silt,  278,  280. 
Sinter,  calcareous,  313. 

"       siliceous,  298. 
Slate,  369. 

"     cleavage  of,  371. 
Soapstone,  378,  391. 
Soda,  test  for,  119. 
Sodalite.  48. 


414 


INDEX 


Soil,  formation  of,  276. 
"     gradation  of,  278. 
"     movement  of,  277. 
Specific  gravity  of  minerals,  30. 
"  "        "  rocks,  401. 

"  "       table  of,  31. 

Specular  iron  ore,  91. 
Spherulites,  264. 
Spinel,  90. 
Staurolite,  76. 
Steatite,  378. 
Stilbite,  103. 
Stocks,  138. 
Stratified  rocks,  275,  293. 

"      chemical  origin  of, 
287. 
Stratified    rocks,  classification   of, 

290. 

Stratified  rocks,  color  of,  286. 
"  "      minerals  of,  290. 

*  "      origin  of,  275. 

"  "      structures  of,  282. 

"  "      texture  of,  284. 

Streak  of  minerals,  25. 
Structures  of  igneous  rocks,  158. 

"  "  metamorphic    rocks, 

340. 

Structures  of  stratified  rocks,  282. 
Sulphuric  acid,  test  for,  121. 
Syenite,  218. 

"        common,  219. 
"        corundum,  226. 
"        nephelite,  221. 
Syenite-porphyry,  243. 

Table  determining  rocks,  404. 

"  "  minerals,  122. 

Talc,  102. 

Talc-schist,  374,  391. 
Test  for  alumina,  117. 

calcium,  119. 

carbonates,  115. 

chlorine,  121. 

fluorine,  121,  81. 

gelatinization,  115. 

iron,  118. 

lime,  119. 

magnesium,  119. 

phosphoric  acid,  122. 

potash,  120. 

silicates,  115. 
"  sodium,  119. 


Test  for  sulphuric  acid,  121. 

"       "    water,  120. 
Texture,  augen,  343. 

even-granular,  155. 
factors  influencing,  151. 
foliated,  341. 

"         igneous     rock,   150,   151. 
154. 
Texture,  lenticular,  341. 

"         metamorphic  rock,  340. 
"         poikilitic,  239. 
"         porphyritic,  156. 
"         pseudo-porphyritic,  342. 
"         relation     to    occurrence 
153. 
"Texture,  schistose,  341. 

"         slaty,  342. 
Thin  sections,  8. 
Tinguaite,  222,  254. 
Topaz,  81. 
Tourmaline,  78. 
Trachyte,  250. 
Trap,  257. 
Travertine,  312. 
Tremolite,  61,  63. 
Tripolite,  298. 
Tro  -tolite,  230. 
Tufa,  calcareous,  312. 
Tuff,  volcanic,  140,  272. 

"         "         origin  of,  269. 
Twinning  of  crystals,  36. 
"  multiple,  38. 
"          use  of,  46. 

Uralite,  66. 

Variation  of  minerals,  142. 
"  diagram  of,  145. 

Vesicular  structure,  158. 
Vesuvianite,  75. 
Vitrophyre,  267. 
Volcanic  tuff,  272. 

"     breccia,  272. 

Wacke,  259. 
Water,  test  for,  120. 
Weathering,  belt  of,  337. 

"  of  rocks,  276. 

Wollastonite-rock,  388. 

Zeolites,  103. 
Zinnwaldite,  52. 
Zoisite,  74. 


A     000652105     8 


